ceph: fix null pointer dereference in discard_cap_releases()

send_mds_reconnect() may call discard_cap_releases() after all
release messages have been dropped by cleanup_cap_releases()
Signed-off-by: Yan, Zheng <zheng.z.yan@intel.com>
Reviewed-by: Sage Weil <sage@inktank.com>

fs/ceph/mds_client.c

@@ -1462,15 +1462,18 @@ static void discard_cap_releases(struct ceph_mds_client *mdsc,
 
 	dout("discard_cap_releases mds%d\n", session->s_mds);
 
-	/* zero out the in-progress message */
-	msg = list_first_entry(&session->s_cap_releases,
-			       struct ceph_msg, list_head);
-	head = msg->front.iov_base;
-	num = le32_to_cpu(head->num);
-	dout("discard_cap_releases mds%d %p %u\n", session->s_mds, msg, num);
-	head->num = cpu_to_le32(0);
-	msg->front.iov_len = sizeof(*head);
-	session->s_num_cap_releases += num;
+	if (!list_empty(&session->s_cap_releases)) {
+		/* zero out the in-progress message */
+		msg = list_first_entry(&session->s_cap_releases,
+					struct ceph_msg, list_head);
+		head = msg->front.iov_base;
+		num = le32_to_cpu(head->num);
+		dout("discard_cap_releases mds%d %p %u\n",
+		     session->s_mds, msg, num);
+		head->num = cpu_to_le32(0);
+		msg->front.iov_len = sizeof(*head);
+		session->s_num_cap_releases += num;
+	}
 
 	/* requeue completed messages */
 	while (!list_empty(&session->s_cap_releases_done)) {