mnt: Make sure we don't hit nil dereference on nested mount

Such case is actually a BUG but while we can resolve
the situation without real bug-on call lets walk in
a gentle way.
Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

mount.c

@@ -359,6 +359,11 @@ static struct mount_info *mnt_build_ids_tree(struct mount_info *list, bool inser
 				 * the main root.
 				 */
 				parent = tmp_root_mount;
+				if (unlikely(!tmp_root_mount)) {
+					pr_err("Nested mount %d (@%s %s) w/o root insertion detected\n",
+					       m->mnt_id, m->mountpoint, m->root);
+					return NULL;
+				}
 
 				pr_debug("Mountpoint %d (@%s) get parent %d (@%s)\n",
 					 m->mnt_id, m->mountpoint,