BUG/MEDIUM: quic: Avoid some crashes upon TX packet allocation failures

BUG/MEDIUM: quic: Avoid some crashes upon TX packet allocation failures

If a TX packet cannot be allocated (by qc_build_pkt()), as it can be coalesced
to another one, this leads the TX buffer to have remaining not sent prepared data.
Then haproxy crashes upon a BUG_ON() triggered by the next call to qc_txb_release().
This may happen only during handshakes.

To fix this, qc_build_pkt() returns a new -3 error to dected such allocation
failures followed which is for now on followed by a call to qc_purge_txbuf() to
send the TX prepared data and purge the TX buffer.

Must be backported as far as 2.6.