check for readonly variables within loops

[robstoll]

For new checks and feature suggestions

• https://www.shellcheck.net/ (i.e. the latest commit) currently gives no useful warnings about this
• I searched through https://github.com/koalaman/shellcheck/issues and didn't find anything related

Here's a snippet or screenshot that shows the problem:

foo() {
  while read -r -d $'\0' file; do
    local -r someVar="..."
  done
}

Here's what shellcheck currently says:

nothing

Here's what I wanted or expected to see:

readonly variable within a loop is smelly as it will fail if the loop is executed more than once