boot: fix vbe_find_first_device()

uclass_find_first_device() may return NULL if no device for the uclass
exists. Handle this case gracefully.

Addresses-Coverity: CID 356244 ("Null pointer dereferences (FORWARD_NULL)")
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Simon Glass <sjg@chromium.org>