message: Fix auto type conversion from size_t to double pointed by AppVeyor
  This is part of import QGroundControl plan mission.
diff: @@ -971,7 +971,7 @@ MissionImpl::compose_mission_items_from_json(Mission::mission_items_t &mission_i
  float gimbal_pitch_deg = 0.f, gimbal_yaw_deg = 0.f;
  bool is_fly_through = false;
  double lat_deg = NAN, lon_deg = NAN, rel_alt_deg = NAN;
  - size_t photo_inteval = 0;
  + double photo_inteval = 0.;
  auto camera_action = MissionItem::CameraAction::NONE;
  auto command = json_mission_item["command"].int_value();
  @@ -983,7 +983,7 @@ MissionImpl::compose_mission_items_from_json(Mission::mission_items_t &mission_i
  switch (command) {
  case MAV_CMD_IMAGE_START_CAPTURE:
  - photo_inteval = static_cast<size_t>(params[1]);
  + photo_inteval = params[1];
  camera_action = MissionItem::CameraAction::START_PHOTO_INTERVAL;
  break;
  case MAV_CMD_VIDEO_START_STREAMING:
  