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

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.