MFC r366198:
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.