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
sctplab/usrsctp#408

MFC after:		1 week


git-svn-id: svn+ssh://svn.freebsd.org/base/head@355172 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f