test/dm: check if devices exist

Running 'ut dm' on the sandbox without -D or -d results in segmentation
faults due to NULL pointer dereferences.

Check that device pointers are non-NULL before using them.

Use ut_assertnonnull() for pointers instead of ut_assert().

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Tested-by: Philippe Reynes <philippe.reynes@softathome.com>

test/dm/devres.c

@@ -153,6 +153,7 @@ static int dm_test_devres_phase(struct unit_test_state *uts)
 	 * allocation created in the bind() method.
 	 */
 	ut_assertok(uclass_find_first_device(UCLASS_TEST_DEVRES, &dev));
+	ut_assertnonnull(dev);
 	devres_get_stats(dev, &stats);
 	ut_asserteq(1, stats.allocs);
 	ut_asserteq(TEST_DEVRES_SIZE, stats.total_size);