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 pion/sctp#138.

MFC after:		3 days

sys/netinet/sctp_indata.c

@@ -5403,7 +5403,9 @@ sctp_flush_reassm_for_str_seq(struct sctp_tcb *stcb,
 	 * it can be delivered... But for now we just dump everything on the
 	 * queue.
 	 */
-	if (!asoc->idata_supported && !ordered && SCTP_TSN_GT(control->fsn_included, cumtsn)) {
+	if (!asoc->idata_supported && !ordered &&
+	    control->first_frag_seen &&
+	    SCTP_TSN_GT(control->fsn_included, cumtsn)) {
 		return;
 	}
 	TAILQ_FOREACH_SAFE(chk, &control->reasm, sctp_next, nchk) {