BUG/MEDIUM: quic: Don't try to use hystart if not implemented

Not every CC algos implement hystart, so only call the method if it is
actually there. Failure to do so will cause crashes if hystart is on,
and the algo doesn't implement it.

This should fix github issue haproxy#3218

This should be backported up to 3.0.

(cherry picked from commit a08bc46)
Signed-off-by: Christopher Faulet <[email protected]>
(cherry picked from commit d848a08)
[cf: ctx adjt]
Signed-off-by: Christopher Faulet <[email protected]>

Author: cognet

src/quic_tx.c

@@ -428,7 +428,8 @@ static int qc_send_ppkts(struct buffer *buf, struct ssl_sock_ctx *ctx)
 			}
 			qc->path->in_flight += pkt->in_flight_len;
 			pkt->pktns->tx.in_flight += pkt->in_flight_len;
-			if ((quic_tune.options & QUIC_TUNE_CC_HYSTART) && pkt->pktns == qc->apktns)
+			if ((quic_tune.options & QUIC_TUNE_CC_HYSTART) && pkt->pktns == qc->apktns &&
+			    cc->algo->hystart_start_round != NULL)
 				cc->algo->hystart_start_round(cc, pkt->pn_node.key);
 			if (pkt->in_flight_len)
 				qc_set_timer(qc);