[APPLETALK]: Fix potential OOPS in atalk_sendmsg().

atrtr_find() can return NULL, so do not blindly dereference
rt->dev before we check for rt being NULL.
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/appletalk/ddp.c

@@ -1584,7 +1584,6 @@ static int atalk_sendmsg(struct kiocb *iocb, struct socket *sock, struct msghdr
 
 	if (usat->sat_addr.s_net || usat->sat_addr.s_node == ATADDR_ANYNODE) {
 		rt = atrtr_find(&usat->sat_addr);
-		dev = rt->dev;
 	} else {
 		struct atalk_addr at_hint;
 
@@ -1592,7 +1591,6 @@ static int atalk_sendmsg(struct kiocb *iocb, struct socket *sock, struct msghdr
 		at_hint.s_net  = at->src_net;
 
 		rt = atrtr_find(&at_hint);
-		dev = rt->dev;
 	}
 	if (!rt)
 		return -ENETUNREACH;