test: pwm: 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: 161690)
Fixes: 43b4156 (dm: sandbox: pwm: Add a basic pwm test)

test/dm/pwm.c

@@ -18,6 +18,7 @@ static int dm_test_pwm_base(struct unit_test_state *uts)
 	struct udevice *dev;
 
 	ut_assertok(uclass_get_device(UCLASS_PWM, 0, &dev));
+	ut_assertnonnull(dev);
 	ut_assertok(pwm_set_config(dev, 0, 100, 50));
 	ut_assertok(pwm_set_enable(dev, 0, true));
 	ut_assertok(pwm_set_enable(dev, 1, true));