Previously the following read and wrote 1 byte out-of-bounds:

    $ valgrind ruby -e 'p /(\W+)[bx]\?/i.match? "aaaaaa aaaaaaaaa aaaa aaaaaaaa aaa aaaaxaaaaaaaaaaa aaaaa aaaaaaaaaaaa a ? aaa aaaa a ?"' 2> >(grep Invalid -A 30)

Because of the `match_cache_point_index + 1` in
memoize_extended_match_cache_point() and
check_extended_match_cache_point(), we need one more byte of space.