BUG/MEDIUM: quic: prevent crash on Retry sending

BUG/MEDIUM: quic: prevent crash on Retry sending

The following commit introduced a regression :
  commit 1a5cc19cecfa84bfd0fdd7b9d5d20899cce40662
  MINOR: quic: adjust Rx packet type parsing

Since this commit, qv variable was left to NULL as version is stored
directly in quic_rx_packet instance. In most cases, this only causes
traces to skip version printing. However, qv is dereferenced when
sending a Retry which causes a segfault.

To fix this, simply remove qv variable and use pkt->version instead,
both for traces and send_retry() invocation.

This bug was detected thanks to QUIC interop runner. It can easily be
reproduced by using quic-force-retry on the bind line.

This must be backported up to 2.7.