Fix parsing error when processing cmsg in SCTP send calls.
Fix parsing error when processing cmsg in SCTP send calls.

The bug is related to a signed/unsigned mismatch.
This should most likely fix the issue in sctp_sosend reported by
Dmitry Vyukov on the freebsd-hackers mailing list and found by
running syzkaller.

sctplab/stream-reset-improved,sctplab/stream-reset-improved