[media] media: vb2: fix queueing of userptr buffers with null buffer pointer

Heuristic that checks if the memory pointer has been changed lacked a check
if the pointer was actually provided by the userspace, what allowed one to
queue a NULL pointer which was accepted without further checking. This
patch fixes this issue.

Reported-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
CC: Pawel Osciak <pawel@osciak.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>