Go back to scanning string from current character

Turns out `patch` wasn't populated yet, so my last commit was scanning
nothing. Finally found the bug: it need to start scanning at `atchar`,
not `len - atchar`. Wild that this never came up before! Only found it
just now by, at the last second, testing the example from #70:
`0.0.0-00010101000000-000000000000`. Now included in the test corpus.

src/semver.c

@@ -214,12 +214,12 @@ semver* parse_semver(char* str, bool lax, bool throw, bool* bad) {
                 if (i >= 1 && (i == 1 || patch[i-2] == '.') && patch[i-1] == '0' && isdigit(next)) {
                     pred = true;
                     // Numeric identifiers must not include a leading 0. Scan ahead.
-                    for (p = i; p < strlen(patch); p++) {
-                        if (patch[p] == '.') {
+                    for (p = atchar; p < len; p++) {
+                        if (str[p] == '.') {
                             // We got to the end of this bit.
                             break;
                         }
-                        if (isalpha(patch[p]) || patch[p] == '-') {
+                        if (isalpha(str[p]) || str[p] == '-') {
                             // If there is a letter or a dash, it's okay to start with a leading 0.
                             pred = false;
                             break;

test/sql/corpus.sql

@@ -6,7 +6,7 @@ BEGIN;
 \i test/pgtap-core.sql
 CREATE EXTENSION semver;
 
-SELECT plan(75);
+SELECT plan(76);
 --SELECT * FROM no_plan();
 
 -- Valid Semantic Versions
@@ -47,7 +47,8 @@ SELECT lives_ok(
     '1.0.0-0010-1234',
     '1.0.0-1.2.3-1234',
     '1.0.0-1234',
-    '1.0.0-4321'
+    '1.0.0-4321',
+    '0.0.0-00010101000000-000000000000'
     -- 
 ]) AS v;