Fix computation of regex stack limit

Fix computation of regex stack limit

The regex stack limit was being computed as the number of stack entries,
whereas it was being compared with the current size as measured in
bytes.  This could cause indefinite looping when nearing the stack limit
if re_max_failures happened not to be a multiple of sizeof
fail_stack_elt_t (Bug #24751).

* src/regex.c (GROW_FAIL_STACK): Compute both current stack size and
limit as numbers of stack entries.