ubifs: Check for name being NULL while mounting

The requested device name can be NULL or an empty string.
Check for that and refuse to continue. UBIFS has to do this manually
since we cannot use mount_bdev(), which checks for this condition.

Fixes: 1e51764a3c ("UBIFS: add new flash file system")
Reported-by: syzbot+38bd0f7865e5c6379280@syzkaller.appspotmail.com
Signed-off-by: Richard Weinberger <richard@nod.at>

fs/ubifs/super.c

@@ -1954,6 +1954,9 @@ static struct ubi_volume_desc *open_ubi(const char *name, int mode)
 	int dev, vol;
 	char *endptr;
 
+	if (!name || !*name)
+		return ERR_PTR(-EINVAL);
+
 	/* First, try to open using the device node path method */
 	ubi = ubi_open_volume_path(name, mode);
 	if (!IS_ERR(ubi))