Fixes leaks when `utf16leToUtf8Alloc`/`utf16leToUtf8AllocZ`/`utf8ToUtf16LeWithNull` return an error and adds relevant test cases
Fixes leaks when `utf16leToUtf8Alloc`/`utf16leToUtf8AllocZ`/`utf8ToUtf16LeWithNull` return an error and adds relevant test cases