This is another bug in computing the fastmap. It was reported by a user
of sed because it usually does not happen with !_LIBC. However, it is
there in that case too.
The bug is that whenever we have a range at the beginning of the regex,
the regex must be tested on any possible multibyte character. The reason
why _LIBC masks it, is that in general there is a collation symbol for
each possible multibyte-character lead byte, so all the lead bytes are
in general already part of the fastmap.
The tests use cyrillic characters as an example. With _LIBC, they pass
without the patch too, but you can make them fail by removing collation
symbols handling.
