Convert some calls to test for malformations

Convert some calls to test for malformations

Code review showed several places in core where a UTF-8 sequence that
was for a code point below 256 could be malformed, and be blindly
accepted.  Convert these to use the similar macro that does the check.

One place in regexec.c was not converted because it is working on the
pattern, which perl should have generated itself, so very unlikely to be
bemalformed.

I didn't add tests for these, as it would be a pain to figure out
somehow to trigger them, and this is precautionary, based on code
reading rather than any known field experience.