BUG/MINOR: quic: fix BUG_ON() on Tx pkt alloc failure

On quic_tx_packet allocation failure, it is possible to trigger BUG_ON()
crash on INITIAL packet building. This statement is responsible to
ensure INITIAL packets are padded to 1.200 bytes as required. If a
packet on higher encryption level allocation fails, PADDING frame cannot
properly encoded, despite the INITIAL packet properly built.

This crash happens due to qc_txb_store() invokation after quic_tx_packet
allocation failure to validate already built packets. However, this
statement is unneeded as qc_purge_tx_buf() is called just after. Simply
remove qc_txb_store() to fix this issue.

This was detected using -dMfail.

This should be backported up to 2.6.

(cherry picked from commit d5376b7)
[cf: ctx adjt]
Signed-off-by: Christopher Faulet <[email protected]>

Author: a-denoyelle

src/quic_tx.c

@@ -601,8 +601,6 @@ static int qc_prep_pkts(struct quic_conn *qc, struct buffer *buf,
 			                       probe, cc, &err);
 			switch (err) {
 				case -3:
-					if (first_pkt)
-						qc_txb_store(buf, dglen, first_pkt);
 					qc_purge_tx_buf(qc, buf);
 					goto leave;
 				case -2: