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: 1e51764a

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