MFS r366324:
Improve the handling of receiving unordered and unreliable user
messages using DATA chunks. Don't use fsn_included when not being
sure that it is set to an appropriate value. If the default is
used, which is -1, this can result in SCTP associaitons not
making any user visible progress.

Thanks to Yutaka Takeda for reporting this issue for the the
userland stack in https://github.com/pion/sctp/issues/138.

MFS r366329:
Improve the input validation and processing of cookies.
This avoids setting the association in an inconsistent
state, which could result in a use-after-free situation.
This can be triggered by a malicious peer, if the peer
can modify the cookie without the local endpoint recognizing
it.
Thanks to Ned Williamson for reporting the issue.

Approved by: re (gjb)