Revert most of 3a8bbffbce: Avoid unnecessary malformed checking

Revert most of 3a8bbffbce: Avoid unnecessary malformed checking

My thinking was muddled when I made that commit, and this reverts the
essence of it.  The theory was that since we have already processed the
regex pattern, we don't need to check it for malformedness, hence we
don't need the "safe" form of certain macros that check for and avoid
running off the end of the buffer.  It is true that we don't have to
worry about malformedness indicating that the buffer is bigger than it
really is, but these macros can match up to three well-formed
characters, so we do have to make sure that all three are in the buffer,
and that the input isn't just the first two at the buffer's end.

This was caught by running valgrind.