BUG/MINOR: quic: fix crash on client handshake abort

BUG/MINOR: quic: fix crash on client handshake abort

On backend side, a connection can be aborted and released prior to
handshake completion. This causes a crash in qc_ssl_do_hanshake() as
<conn> member of ssl_sock_ctx is not reset in this case.

To fix this, use <conn> member of quic_conn instead. This is safe as it
is properly set to NULL when a connection is released.

No impact on the frontend side as <conn> member is not accessed. Indeed,
in this case connection is most of the times allocated after handshake
completion.

No need to be backported.