Changes

src/stream.c: Use sctp_assoc_id 'canary' to detect misisng sinfo

sctp_rcvinfo() has a successful return path that does *not* fill
the caller-provided sinfo structure.  This would be highly unexpected
as we cannot determine the stream_id and PPID then.

Our osmo_io code path already handles that situation correctly
by logging a related error message.  The non-osmo-io path however
would silently hide that error.

See also: https://github.com/sctp/lksctp-tools/issues/37

Change-Id: Idf9c0fa4db65ab6ea34a9cb2011400aadf3dd54e