ax25: Fix NULL pointer dereference and lockup.

From: Jarek Poplawski <jarkao2@gmail.com>

There is only one function in AX25 calling skb_append(), and it really
looks suspicious: appends skb after previously enqueued one, but in
the meantime this previous skb could be removed from the queue.

This patch Fixes it the simple way, so this is not fully compatible with
the current method, but testing hasn't shown any problems.

Signed-off-by: Ralf Baechle <ralf@linux-mips.org>
Signed-off-by: David S. Miller <davem@davemloft.net>