V4L/DVB (11991): buf-core.c: add pointer check

add poiter check for videobuf_queue_core_init().

any guys who write a v4l driver, pass a NULL pointer or a non-inintial
pointer to the first parameter such as videobuf_queue_sg_init() , it
would be crashed.

Signed-off-by: Figo.zhang <figo1802@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>