[media] media: vb2: add a check for uninitialized buffer

__buffer_in_use() might be called for empty/uninitialized buffer in the
following scenario: REQBUF(n, USER_PTR), QUERYBUF(). This patch fixes
kernel ops in such case.

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>