[media] vb2: Fix error handling in '__vb2_buf_mem_alloc'

'call_ptr_memop' can return NULL, so we must test its return value with
'IS_ERR_OR_NULL'. Otherwise, the test 'if (mem_priv)' is meaningless.
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: Hans Verkuil <hansverk@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>

drivers/media/v4l2-core/videobuf2-core.c

@@ -210,7 +210,7 @@ static int __vb2_buf_mem_alloc(struct vb2_buffer *vb)
 		mem_priv = call_ptr_memop(vb, alloc,
 				q->alloc_devs[plane] ? : q->dev,
 				q->dma_attrs, size, dma_dir, q->gfp_flags);
-		if (IS_ERR(mem_priv)) {
+		if (IS_ERR_OR_NULL(mem_priv)) {
 			if (mem_priv)
 				ret = PTR_ERR(mem_priv);
 			goto free;