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

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

This back-patches commit d9f37e666 into out-of-support branches,
pursuant to newly-established project policy.  The point is to
suppress "uninitialized variable" warnings so that people building
these branches needn't expend brain cells verifying that it's safe
to ignore the warnings.

Discussion: https://postgr.es/m/d0316012-ece7-7b7e-2d36-9c38cb77cb3b@enterprisedb.com