App receives packets from the wrong source address on a group+port collision.

I noticed this while running MNAT on the receiver.  Packets arriving on the physical interface with a different source address than the subscribed (S,G) were received instead of filtered in the library.  (The OS can't filter it because the listening UDP socket is only bound to the group and the port; even though the sockopt is an SSM join, that's not a filter for the receiving port.)