media: videobuf2: Exit promptly if size = 0

Exit as soon as possible if a driver wants to allocate a buffer of
size 0, and print a warning to help fixing the issue.

Signed-off-by: Ricardo Ribalda <ribalda@chromium.org>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>