mon/MonClient: do not assert on auth in build_authorizer()

mon/MonClient: do not assert on auth in build_authorizer()

* there is chance that some connections is still trying to authorize
  itself after the MonClient is shut down. do not assert in this case,
  but it is a sign of bug, or bad shutdown sequence, so print a message to
  dout().
* do not use active_con->get_auth() as an alternative to `this->auth` if
  it is not available. because we promote the authorized conn in
  pending_cons as the active_con, and std::swap(active_conn->auth, this->auth)
  with the monc_lock. so there is no point to return active_con->get_auth(),
  as it's always null.

Signed-off-by: Kefu Chai <kchai@redhat.com>