Don't access pointers to freed storage in regex.c

Don't access pointers to freed storage in regex.c

Remove __BOUNDED_POINTERS__ code, which does not work with
-fcheck-pointer-bound and which has undefined behavior anyway.
Problem found when trying to port to gcc -fcheck-pointer-bounds.
(This code was removed from glibc and gnulib regex.c many years ago.)
* src/regex.c (ELSE_EXTEND_BUFFER_HIGH_BOUND): Remove.
(EXTEND_BUFFER): Use a more-portable approach that avoids
undefined behavior due to inspecting pointers to freed storage.