Regular expression execution can panic

[Razican]

The Function::add_rest_param() function can panic with the following error message:

thread 'main' panicked at 'Could not get slice', boa/src/builtins/regexp/mod.rs:413:56

This has been observed when running this ECMAScript test.

Interestingly this only happens when executing the full test suite, and not if we ask the backtrace or run the test individually.