message: mission: don't set a loiter item with 0 time
  Presumably this lead to strange flight behaviour if set to 0 together
  with non-fly-through.
diff: @@ -583,7 +583,10 @@ void MissionImpl::assemble_mavlink_messages()
  // FIXME: It is a bit of a hack to set a LOITER_TIME waypoint to add a delay.
  // A better solution would be to properly use NAV_DELAY instead. This
  // would not require us to keep the last lat/lon.
  - if (std::isfinite(mission_item_impl.get_loiter_time_s())) {
  + // A loiter time of NAN is ignored but also a loiter time of 0 doesn't
  + // make any sense and should be discarded.
  + if (std::isfinite(mission_item_impl.get_loiter_time_s()) &&
  + mission_item_impl.get_loiter_time_s() > 0.0f) {
  if (!last_position_valid) {
  // In the case where we get a delay without a previous position, we will have to
  // ignore it.
  