ipv4: Fix crash in dst_release when udp_sendmsg route lookup fails.

As reported by Eric:

[11483.697233] IP: [<c12b0638>] dst_release+0x18/0x60
 ...
[11483.697741] Call Trace:
[11483.697764]  [<c12fc9d2>] udp_sendmsg+0x282/0x6e0
[11483.697790]  [<c12a1c01>] ? memcpy_toiovec+0x51/0x70
[11483.697818]  [<c12dbd90>] ? ip_generic_getfrag+0x0/0xb0

The pointer passed to dst_release() is -EINVAL, that's because
we leave an error pointer in the local variable "rt" by accident.

NULL it out to fix the bug.
Reported-by: NEric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv4/udp.c

@@ -925,6 +925,7 @@ int udp_sendmsg(struct kiocb *iocb, struct sock *sk, struct msghdr *msg,
 		rt = ip_route_output_flow(net, &fl, sk);
 		if (IS_ERR(rt)) {
 			err = PTR_ERR(rt);
+			rt = NULL;
 			if (err == -ENETUNREACH)
 				IP_INC_STATS_BH(net, IPSTATS_MIB_OUTNOROUTES);
 			goto out;