BUG/MEDIUM: quic: fix FD for quic_cc_conn

BUG/MEDIUM: quic: fix FD for quic_cc_conn

Since following commit, quic_conn closes its owned socket before
transition to quic_cc_conn for closing state. This allows to save FDs as
quic_cc_conn could use the listener socket for their I/O.

  commit 150c0da8895be50a39fd8e44f1db28e52c938569
  MEDIUM: quic: release conn socket before using quic_cc_conn

This patch is incomplete as it removes initialization of <fd> member for
quic_cc_conn. Thus, if sending is done on closing state, <fd> value is
undefined which in most cases will result in a crash. Fix this by simply
initializing <fd> member with qc_init_fd() in qc_new_cc_conn().

This bug should fix recent issue from #2095. Thanks to Tristan for its
reporting and then testing of this patch.

No need to backport.