ubifs: skip dumping tnc tree when zroot is null

Clearing slab cache will free all znode in memory and make
c->zroot.znode = NULL, then dumping tnc tree will access
c->zroot.znode which cause null pointer dereference.

Link: https://bugzilla.kernel.org/show_bug.cgi?id=219624#c0
Fixes: 1e51764a3c2a ("UBIFS: add new flash file system")
Signed-off-by: pangliyuan <[e-mail]>
Reviewed-by: Zhihao Cheng <[e-mail]>
Signed-off-by: Richard Weinberger <[e-mail]>

fs/ubifs/debug.c

···
 
 	pr_err("\n");
 	pr_err("(pid %d) start dumping TNC tree\n", current->pid);
-	znode = ubifs_tnc_levelorder_next(c, c->zroot.znode, NULL);
-	level = znode->level;
-	pr_err("== Level %d ==\n", level);
-	while (znode) {
-		if (level != znode->level) {
-			level = znode->level;
-			pr_err("== Level %d ==\n", level);
+	if (c->zroot.znode) {
+		znode = ubifs_tnc_levelorder_next(c, c->zroot.znode, NULL);
+		level = znode->level;
+		pr_err("== Level %d ==\n", level);
+		while (znode) {
+			if (level != znode->level) {
+				level = znode->level;
+				pr_err("== Level %d ==\n", level);
+			}
+			ubifs_dump_znode(c, znode);
+			znode = ubifs_tnc_levelorder_next(c, c->zroot.znode, znode);
 		}
-		ubifs_dump_znode(c, znode);
-		znode = ubifs_tnc_levelorder_next(c, c->zroot.znode, znode);
+	} else {
+		pr_err("empty TNC tree in memory\n");
 	}
 	pr_err("(pid %d) finish dumping TNC tree\n", current->pid);
 }