test: wdt: Add a check that dev is not NULL

We know that uclass_get_device() does not return NULL for dev when it
succeeds but coverity does not. Add an extra check to hopefully keep it
happy.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reported-by: Coverity (CID: 163247)
Fixes: 0753bc2 (dm: Simple Watchdog uclass)

test/dm/wdt.c

@@ -20,6 +20,7 @@ static int dm_test_wdt_base(struct unit_test_state *uts)
 	const u64 timeout = 42;
 
 	ut_assertok(uclass_get_device(UCLASS_WDT, 0, &dev));
+	ut_assertnonnull(dev);
 	ut_asserteq(0, state->wdt.counter);
 	ut_asserteq(false, state->wdt.running);