regexec.c: Fix "\x{FB01}\x{FB00}" =~ /ff/i

regexec.c: Fix "\x{FB01}\x{FB00}" =~ /ff/i

Only the first character of the string was being checked when scanning
for the beginning position of the pattern match.

This was so wrong, it looks like it has to be a regression.  I
experimented a little and did not find any.  I believe (but am not
certain) that a multi-char fold has to be involved.  The the handling of
these was so broken before 5.14 that there very well may not be a
regression.