Avoid crash if extrainfo generation fails

If we fail to generate an extrainfo (can that happen?) we go on generating a routerinfo, but later in router_rebuild_descriptor() we do:

  if (options->BridgeRelay) {
    /* Bridges shouldn't be able to send their descriptors unencrypted,
       anyway, since they don't have a DirPort, and always connect to the
       bridge authority anonymously.  But just in case they somehow think of
       sending them on an unencrypted connection, don't allow them to try. */
    ri->cache_info.send_unencrypted = ei->cache_info.send_unencrypted = 0;
  } else {
    ri->cache_info.send_unencrypted = ei->cache_info.send_unencrypted = 1;
  }

note the unconditional assignment to a field of ei.