Fix a potential NULL pointer dereference by skipping registration of
external entities in case none are provided.

This is useful at least when testing mere memory-to-memory scenarios.
Signed-off-by: NOhad Ben-Cohen <ohad@wizery.com>
Acked-by: NLaurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: NMauro Carvalho Chehab <mchehab@redhat.com>