Fix potential integer overflow in GC_find_limit_* functions

Fix potential integer overflow in GC_find_limit_* functions

* os_dep.c [OPENBSD] (GC_find_limit_openbsd, GC_skip_hole_openbsd): Add
assertion for the minimum value of bound; increment result by pgsz only
if no overflow is guaranteed.
* os_dep.c [USE_PROC_FOR_LIBRARIES && THREADS || NEED_FIND_LIMIT]
(GC_find_limit_with_bound): Add assertion for the minimum/maximum value
of bound; increment/decrement result by MIN_PAGE_SIZE only if no
overflow/underflow is guaranteed.