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

We know that uclass_get_device() and device_find_child_by_of_offset() do
not return NULL for dev when they 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: 163246)
Fixes: 0753bc2d (dm: Simple Watchdog uclass)