net/rose: fix NULL ax25_cb kernel panic

When an internally generated frame is handled by rose_xmit(),
rose_route_frame() is called:

        if (!rose_route_frame(skb, NULL)) {
                dev_kfree_skb(skb);
                stats->tx_errors++;
                return NETDEV_TX_OK;
        }

We have the same code sequence in Net/Rom where an internally generated
frame is handled by nr_xmit() calling nr_route_frame(skb, NULL).
However, in this function NULL argument is tested while it is not in
rose_route_frame().
Then kernel panic occurs later on when calling ax25cmp() with a NULL
ax25_cb argument as reported many times and recently with syzbot.

We need to test if ax25 is NULL before using it.

Testing:
Built kernel with CONFIG_ROSE=y.

Signed-off-by: Bernard Pidoux <f6bvp@free.fr>
Acked-by: Dmitry Vyukov <dvyukov@google.com>
Reported-by:  <syzbot+1a2c456a1ea08fa5b5f7@syzkaller.appspotmail.com>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: Bernard Pidoux <f6bvp@free.fr>
Cc: linux-hams@vger.kernel.org
Cc: netdev@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Signed-off-by: David S. Miller <davem@davemloft.net>