Add checks for valid multibyte character length in UtfToLocal, LocalToUtf.

This is mainly to suppress "uninitialized variable" warnings from very
recent versions of gcc.  But it seems like a good robustness thing anyway,
not to mention that we might someday decide to support 6-byte UTF8.

Per report from Karol Trzcionka.  No back-patch since there's no reason
at the moment to think this is more than cosmetic.