[media] videobuf2: fix core to correctly identify allocation failures

The videobuf2-dma-contig allocator returns an ERR_PTR() on failure, not
a NULL like all other allocators. Fix videobuf2-core to also correctly
recognise those failures.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>