fix: invalid unicode input results in endless loop #17

[sebthom]

This is based on #21 It only adds the minimum modifications required to make the newly added test case succeed.

[headius]

@lopex Perhaps this is another case where the macros should be used to check result?

[headius]

We're still unsure about this issue. It is reproducible with the example from #17 but we're unsure if this is the best fix. It's not possible to reproduce in Ruby because the bad UTF-8 gets rejected before it ever gets into a regex. If we could reproduce with Onigmo, we could file the issue with them (or with ruby-core) to see how they would fix it.

[headius]

I experimented with checking for return value of codeToMbc less than zero and that did not fix the issue (still hangs in a loop). Interestingly, the fix in this PR solves the hanging loop, but fails a few other tests locally (unsure if this is related to recent changes or something I did).