BUG/MINOR: quic: Missing padding in very short probe packets

BUG/MINOR: quic: Missing padding in very short probe packets

This bug arrived with this commit:
   MINOR: quic: Send PING frames when probing Initial packet number space

This may happen when haproxy needs to probe the peer with very short packets
(only one PING frame). In this case, the packet must be padded. There was clearly
a case which was removed by the mentionned commit above. That said, there was
an extra byte which was added to the PADDING frame before the mentionned commit
above. This is no more the case with this patch.

Thank you to @tatsuhiro-t (ngtcp2 manager) for having reported this issue which
was revealed by the keyupdate test (on client side).

Must be backported to 2.7 and 2.6.