Fix race condition - the log must be under lock

As information from the alias strucuture is printed, this must be done under lock protection.
Closes #589 by @dbeskoek.

(cherry picked from commit 22f4c1a)

net/net_tcp.c

@@ -762,10 +762,10 @@ int tcpconn_add_alias(int id, int port, int proto)
 	LM_ERR("no connection found for id %d\n",id);
 	return -1;
 error_sec:
-	TCPCONN_UNLOCK(id);
 	LM_WARN("possible port hijack attempt\n");
 	LM_WARN("alias already present and points to another connection "
 			"(%d : %d and %d : %d)\n", a->parent->id,  port, c->id, port);
+	TCPCONN_UNLOCK(id);
 	return -1;
 }