LibRegex: Ensure nullable quantifiers backtrack when input remains

[aplefull]

This makes patterns like /(a?b??)*/ correctly match the string.

Issue was originally found in test262: https://github.com/tc39/test262/blob/9d8efae5c2c1848b9c3362c4aa81dd360ff8d3a7/test/built-ins/RegExp/nullable-quantifier.js