access_output/udp.c: Fixed an old memleak.

This leak is triggered each time the udp access out is started, so it 
could be the cause of some of these "slow memleak with udp streaming" 
reports we have had, if these users where streaming a file in repeat

modules/access_output/udp.c

@@ -331,6 +331,7 @@ static void Close( vlc_object_t * p_this )
 
     net_Close( p_sys->p_thread->i_handle );
 
+    vlc_object_destroy( p_sys->p_thread );
     /* update p_sout->i_out_pace_nocontrol */
     p_access->p_sout->i_out_pace_nocontrol--;