When the network doesn't have any exits, use the mid weight as the exit weight

If we're building onion service paths, all our circuit ends will be mids.

  /* if the consensus has no exits, treat the exit fraction as 100% */
  if (router_have_consensus_path() != CONSENSUS_PATH_EXIT) {
    f_exit = 1.0;
  }

Bug fix on 55ad54e0146 in 0.2.6.2-alpha.