BUG/MAJOR: quic: Possible crash with full congestion control window

BUG/MAJOR: quic: Possible crash with full congestion control window

This commit reverts this one:
  "d5066dd9d BUG/MEDIUM: quic: qc_prep_app_pkts() retries on qc_build_pkt() failures"

After having filled the congestion control window, qc_build_pkt() always fails.
Then depending on the relative position of the writer and  reader indexes for the
TX buffer, this could lead this function to try to reuse the buffer even if not full.
In such case, we do not always mark the end of the data in this TX buffer. This
is something the reader cannot understand: it reads a false datagram length,
then a wrong packet address from the TX buffer, leading to an invalid pointer
dereferencing.