regen/regcharclass.pl: Forbid non-safe macros for multi-char matches

regen/regcharclass.pl: Forbid non-safe macros for multi-char matches

For matches that can match more than a single code point, one should
always use a macro that makes sure that one doesn't read off the end of
the buffer.  This is because the buffer might end with the first N
characters of a sequence with at least N+1 in it, and we don't want to
read that N+1 position in the buffer.

If this had been in place, buggy commit 3a8bbffbce would not have
happened.