[IPV4]: Possible leak of multicast source filter sctructure

There is a leak of a socket's multicast source filter list structure
on closing a socket with a multicast source filter set on an interface
that does not exist any more.
Signed-off-by: Michal Ruzicka <michal.ruzicka@comstar.cz>
Acked-by: David L Stevens <dlstevens@us.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/igmp.c

@@ -2199,13 +2199,13 @@ void ip_mc_drop_socket(struct sock *sk)
 		struct in_device *in_dev;
 		inet->mc_list = iml->next;
 
-		if ((in_dev = inetdev_by_index(iml->multi.imr_ifindex)) != NULL) {
+		in_dev = inetdev_by_index(iml->multi.imr_ifindex);
 		(void) ip_mc_leave_src(sk, iml, in_dev);
+		if (in_dev != NULL) {
 			ip_mc_dec_group(in_dev, iml->multi.imr_multiaddr.s_addr);
 			in_dev_put(in_dev);
 		}
 		sock_kfree_s(sk, iml, sizeof(*iml));
-
 	}
 	rtnl_unlock();
 }