Fix compiler extra warnings.

Fix compiler extra warnings.

* src/atomic_ops_stack.c (AO_stack_pop_explicit_aux_acquire): Remove
non-negative-value test for an unsigned value.
* src/atomic_ops_stack.h (AO_STACK_INITIALIZER): Add {} to match type
definition.
* tests/test_atomic.c (acqrel_thr): Fix code indentation.
* tests/test_atomic.c (test_and_set_thr): Print thread Id in case of error.
* tests/test_malloc.c (cons): Declare "i" local variable as unsigned (as it
is compared to another unsigned value).
* tests/test_malloc.c (check_list): Test upper bound as well, abort with the
appropriate message in case of mismatch.
* tests/test_malloc.c (run_one_test): Return "arg" parameter (to suppress
"unused parameter" compiler warning).