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

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.