Use EXPECT in malloc result testing for NULL

Use EXPECT in malloc result testing for NULL

* gcj_mlc.c (GC_gcj_malloc_ignore_off_page): Use EXPECT to test "op"
value for NULL.
* malloc.c (GC_generic_malloc_inner, GC_malloc_uncollectable):
Likewise.
* mallocx.c (GC_generic_malloc_many, GC_posix_memalign): Likewise.
* typd_mlc.c (GC_malloc_explicitly_typed,
GC_malloc_explicitly_typed_ignore_off_page,
GC_calloc_explicitly_typed): Likewise.