BUG/MINOR: quic: Endless loop during retransmissions

BUG/MINOR: quic: Endless loop during retransmissions

qc_dgrams_retransmit() could reuse the same local list and could splice it two
times to the packet number space list of frame to be send/resend. This creates a
loop in this list and makes qc_build_frms() possibly endlessly loop when trying
to build frames from the packet number space list of frames. Then haproxy aborts.

This issue could be easily reproduced patching qc_build_frms() function to set <dlen>
variable value to 0 after having built at least 10 CRYPTO frames and using ngtcp2
as client with 30% packet loss in both direction.

Thank you to @gabrieltz for having reported this issue in GH #1903.

Must be backported to 2.6.