rose: Fix Null pointer dereference in rose_send_frame()

rose_send_frame() dereferences `neigh->dev` when called from
rose_transmit_clear_request(), and the first occurrence of the
`neigh` is in rose_loopback_timer() as `rose_loopback_neigh`,
and it is initialized in rose_add_loopback_neigh() as NULL.
i.e when `rose_loopback_neigh` used in rose_loopback_timer()
its `->dev` was still NULL and rose_loopback_timer() was calling
rose_rx_call_request() without checking for NULL.

- net/rose/rose_link.c
This bug seems to get triggered in this line:

rose_call = (ax25_address *)neigh->dev->dev_addr;

Fix it by adding NULL checking for `rose_loopback_neigh->dev`
in rose_loopback_timer().

Fixes: 1da177e4 ("Linux-2.6.12-rc2")
Suggested-by: Jakub Kicinski <kuba@kernel.org>
Reported-by: syzbot+a1c743815982d9496393@syzkaller.appspotmail.com
Tested-by: syzbot+a1c743815982d9496393@syzkaller.appspotmail.com
Link: https://syzkaller.appspot.com/bug?id=9d2a7ca8c7f2e4b682c97578dfa3f236258300b3Signed-off-by: Anmol Karn <anmol.karan123@gmail.com>
Link: https://lore.kernel.org/r/20201119191043.28813-1-anmol.karan123@gmail.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

net/rose/rose_loopback.c

@@ -96,10 +96,19 @@ static void rose_loopback_timer(struct timer_list *unused)
 		}
 
 		if (frametype == ROSE_CALL_REQUEST) {
-			if ((dev = rose_dev_get(dest)) != NULL) {
-				if (rose_rx_call_request(skb, dev, rose_loopback_neigh, lci_o) == 0)
+			if (!rose_loopback_neigh->dev) {
 				kfree_skb(skb);
-			} else {
+				continue;
+			}
+
+			dev = rose_dev_get(dest);
+			if (!dev) {
+				kfree_skb(skb);
+				continue;
+			}
+
+			if (rose_rx_call_request(skb, dev, rose_loopback_neigh, lci_o) == 0) {
+				dev_put(dev);
 				kfree_skb(skb);
 			}
 		} else {