MFC r355172: Ignore assoc IDs on 1-to-1 style SCTP sockets.

Really ignore the SCTP association identifier on 1-to-1 style sockets
as requiresd by the socket API specification.
Thanks to Inaki Baz Castillo, who found this bug running the userland
stack with valgrind and reported the issue in
https://github.com/sctplab/usrsctp/issues/408