BUG/MAJOR: quic: fix padding with short packets

BUG/MAJOR: quic: fix padding with short packets

QUIC sending functions were extended to be more flexible. Of all the
changes, they support now iterating over a variable instance of QEL
instance of only 2 previously. This change has rendered PADDING emission
less previsible, which was adjusted via the following patch :

  a60609f1aa3e5f61d2a2286fdb40ebf6936a80ee
  BUG/MINOR: quic: fix padding of INITIAL packets

Its main purpose was to ensure PADDING would only be generated for the
last iterated QEL instance, to avoid unnecessary padding. In parallel, a
BUG_ON() statement ensure that built INITIAL packets are always padded
to 1.200 bytes as necessary before emitted them.

This BUG_ON() statement caused crash in one particular occurence : when
building datagrams that mixed Initial long packets and 1-RTT short
packets. This last occurence type does not have a length field in its
header, contrary to Long packets. This caused a miscalculation for the
necessary padding size, with INITIAL packets not padded enough to reach
the necessary 1.200 bytes size.

This issue was detected on 3.0.2. It can be reproduced by using 0-RTT
combined with latency. Here are the used commands :

  $ ngtcp2-client --tp-file=/tmp/ngtcp2-tp.txt \
    --session-file=/tmp/ngtcp2-session.txt --exit-on-all-streams-close \
    127.0.0.1 20443 "https://[::]/?s=32o"
  $ sudo tc qdisc add dev lo root netem latency 500ms

Note that this issue cannot be reproduced on current dev version.
Indeed, it seems that the following patch introduce a slight change in
packet building ordering :

  cdfceb10ae136b02e51f9bb346321cf0045d58e0
  MINOR: quic: refactor qc_prep_pkts() loop

This must be backported to 3.0.

This should fix github issue #2609.