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: 0753bc2d (dm: Simple Watchdog uclass)