Nil reference in if statement panics

[sam-at-luther]

Outside of an if statement raymond will silently ignore invalid references within a template. Within the condition of an if, raymond will panic on an invalid reference:

Example:

package main

import (
	"fmt"

	"github.com/aymerick/raymond"
)

func main() {
	tpl := `{{#if wat[0].fnord}}wat{{/if}}`

	ctx := map[string]string{}

	result, err := raymond.Render(tpl, ctx)
	if err != nil {
		panic("Please report a bug :)")
	}

	fmt.Print(result)
}

Results in:

panic: runtime error: invalid memory address or nil pointer dereference [recovered]                                                    
        panic: runtime error: invalid memory address or nil pointer dereference                                                        
[signal SIGSEGV: segmentation violation code=0x1 addr=0x20 pc=0x1103ed6]                                                               
                                                                                                                                       
goroutine 1 [running]:                                                                                                                 
github.com/aymerick/raymond.errRecover(0xc00009fee0)                                                                                   
        /Users/swood/work/src/github.com/aymerick/raymond/template.go:232 +0xf5                                                        
panic(0x112e360, 0x1258f40)                                                                                                            
        /usr/local/Cellar/go/1.13.4/libexec/src/runtime/panic.go:679 +0x1b2                           
github.com/aymerick/raymond.(*Options).HashProp(...)                                                                                   
        /Users/swood/work/src/github.com/aymerick/raymond/helper.go:145                                                                
github.com/aymerick/raymond.(*Options).isIncludableZero(...)                                                                           
        /Users/swood/work/src/github.com/aymerick/raymond/helper.go:281                                                                
github.com/aymerick/raymond.ifHelper(0x0, 0x0, 0x0, 0x0, 0x0)                                                                          
        /Users/swood/work/src/github.com/aymerick/raymond/helper.go:298 +0x26                                                          
reflect.Value.call(0x112a860, 0x115d508, 0x13, 0x11549b2, 0x4, 0xc0000672f0, 0x2, 0x2, 0xc00009f970, 0xc00009f970, ...)
        /usr/local/Cellar/go/1.13.4/libexec/src/reflect/value.go:460 +0x5f6                              
reflect.Value.Call(0x112a860, 0x115d508, 0x13, 0xc0000672f0, 0x2, 0x2, 0x3, 0x1159af0, 0x203000)
        /usr/local/Cellar/go/1.13.4/libexec/src/reflect/value.go:321 +0xb4                                                             
github.com/aymerick/raymond.(*evalVisitor).callFunc(0xc0000c6000, 0x1159aea, 0x2, 0x112a860, 0x115d508, 0x13, 0xc0000672c0, 0x115d508, 0x13, 0x0)
        /Users/swood/work/src/github.com/aymerick/raymond/eval.go:642 +0x46e
github.com/aymerick/raymond.(*evalVisitor).callHelper(0xc0000c6000, 0x1159aea, 0x2, 0x112a860, 0x115d508, 0x13, 0xc00008eb90, 0x1, 0x0) 
        /Users/swood/work/src/github.com/aymerick/raymond/eval.go:649 +0x8c                                                            
github.com/aymerick/raymond.(*evalVisitor).VisitExpression(0xc0000c6000, 0xc00008eb90, 0x0, 0x0)                                       
        /Users/swood/work/src/github.com/aymerick/raymond/eval.go:916 +0x32d                                                           
github.com/aymerick/raymond/ast.(*Expression).Accept(...)                                                                              
        /Users/swood/work/src/github.com/aymerick/raymond/ast/node.go:400                                                              
github.com/aymerick/raymond.(*evalVisitor).VisitBlock(0xc0000c6000, 0xc00008eb40, 0xc00008a2b5, 0x1260f20)
        /Users/swood/work/src/github.com/aymerick/raymond/eval.go:822 +0xb0                                                            
github.com/aymerick/raymond/ast.(*BlockStatement).Accept(0xc00008eb40, 0x1182480, 0xc0000c6000, 0x1148dc0, 0xc00009fdc8)
        /Users/swood/work/src/github.com/aymerick/raymond/ast/node.go:259 +0x3b                                                        
github.com/aymerick/raymond.(*evalVisitor).VisitProgram(0xc0000c6000, 0xc000086f00, 0xc000066f90, 0x0)
        /Users/swood/work/src/github.com/aymerick/raymond/eval.go:783 +0x176                                                           
github.com/aymerick/raymond/ast.(*Program).Accept(...)                                                                                 
        /Users/swood/work/src/github.com/aymerick/raymond/ast/node.go:181                                                              
github.com/aymerick/raymond.(*Template).ExecWith(0xc000088500, 0x112b8e0, 0xc000066f90, 0x0, 0x0, 0x0, 0x0, 0x0)                       
        /Users/swood/work/src/github.com/aymerick/raymond/template.go:220 +0xf3                                                        
github.com/aymerick/raymond.(*Template).Exec(...)                                                                                      
        /Users/swood/work/src/github.com/aymerick/raymond/template.go:194
github.com/aymerick/raymond.Render(0x1159ae7, 0x1e, 0x112b8e0, 0xc000066f90, 0xc000062058, 0x0, 0x0, 0x0)                              
        /Users/swood/work/src/github.com/aymerick/raymond/raymond.go:15 +0x78                                                          
main.main()                                                                                                                            
        /tmp/raymondpanic/main.go:14 +0x54