media: videobuf2: Return error after allocation failure

There is no point to continuing assignment after memory allocation
failed, rather throw error immediately.

Signed-off-by: Souptick Joarder <jrdr.linux@gmail.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
[hverkuil-cisco@xs4all.nl: rebase and remove empty line before the if]
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>