sctp: Clear assoc socket references when freeing a PCB

[markj]

Summary

This restores behaviour present in the first import of SCTP. Commit
ceaad40ae729dea2c5d8ffcfdd45bb96fb8969d2 commented this out and commit
62fb761ff28bb184a2543e539dd689fefd5d3246 removed it. However, once
sctp_inpcb_free() returns, the socket reference is gone no matter what,
so we need to clear it. In particular, it's possible for
sctp_free_assoc() to set SCTP_STATE_ABOUT_TO_BE_FREED and then return
without actually freeing the association.

Reported by: [e-mail]