Fix null pointer dereference on out-of-memory in tests

Fix null pointer dereference on out-of-memory in tests

* tests/disclaim_bench.c (main): Check GC_MALLOC result for NULL
(abort in case of out-of-memory) before dereference.
* tests/test.c (run_one_test): Likewise.
* tests/disclaim_test.c (pair_new): Move is_pair call (in my_assert)
down to be after GC_finalized_malloc result check for NULL.