Fix 'comparison of non-null parameter is always false' warning (Clang)

Fix 'comparison of non-null parameter is always false' warning (Clang)

* alloc.c (GC_set_stop_func, GC_try_to_collect): Use
NONNULL_ARG_NOT_NULL to check argument marked as non-null (in
function declaration) is actually non-null.
* finalize.c (GC_general_register_disappearing_link,
GC_register_long_link, GC_move_disappearing_link, GC_move_long_link):
Likewise.
* misc.c (GC_set_warn_proc, GC_set_abort_func, GC_set_oom_fn): Likewise.
* include/private/gc_priv.h (NONNULL_ARG_NOT_NULL): New macro.