[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>

drivers/media/video/videobuf2-core.c

@@ -51,7 +51,7 @@ static int __vb2_buf_mem_alloc(struct vb2_buffer *vb,
 	for (plane = 0; plane < vb->num_planes; ++plane) {
 		mem_priv = call_memop(q, plane, alloc, q->alloc_ctx[plane],
 					plane_sizes[plane]);
-		if (!mem_priv)
+		if (IS_ERR_OR_NULL(mem_priv))
 			goto free;
 
 		/* Associate allocator private data with this plane */