regexec.c: Guard against malformed UTF-8 in [...]

regexec.c: Guard against malformed UTF-8 in [...]

The code that handles bracketed character classes assumed that the
string being matched against did not have the too-short malformation;
this could lead to reading beyond-the-end-of-buffer.  (It did check for
other malformations.)  This is solved by changing the function that
operates on bracketed character classes to take and use an extra
parameter, the actaul buffer end.