regmatch(): fix out bounds array access

regmatch(): fix out bounds array access

The code for EXACTF and similar tests that

    UCHARAT(s) != fold_array[nextchr]

but doesn't check first that nextchr != NEXTCHR_EOS (-10), so it can
access the byte 10 bytes before the start of one of the PL_fold_latin1 or
similar arrays. Although undesirable, it's harmless, as the worst it can
achieve is a false positive match of the first char of the EXACTF string,
which will then still fail on a full compare of the string.