regexec: Fix off-by-one bug in weight comparison [BZ #23036]

Each weight is prefixed by its length, and the length does not include
itself in the count.  This can be seen clearly from the find_idx
function in string/strxfrm_l.c, for example.  The old code behaved as if
the length itself counted, thus comparing an additional byte after the
weight, leading to spurious comparison failures and incorrect further
partitioning of character equivalence classes.

(cherry picked from commit 7b2f4ced)