drbd: Null pointer deref fix to the large "multi bio rewrite"

Signed-off-by: Philipp Reisner <philipp.reisner@linbit.com>
Signed-off-by: Lars Ellenberg <lars.ellenberg@linbit.com>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>

drivers/block/drbd/drbd_receiver.c

@@ -98,6 +98,10 @@ static struct page *page_chain_del(struct page **head, int n)
 	BUG_ON(!head);
 
 	page = *head;
+
+	if (!page)
+		return NULL;
+
 	while (page) {
 		tmp = page_chain_next(page);
 		if (--n == 0)