message: telemetry: handle lat = 0 lon = 0 correctly
  For the case where the altitude is known but the global position is not,
  the message is sent with lat = 0 and lon = 0.
diff: @@ -512,8 +512,14 @@ void TelemetryImpl::process_global_position_int(const mavlink_message_t& message
  {
  Telemetry::Position position;
  + // Special case to signal lat/lon is not set yet.
  + if (global_position_int.lat == 0 && global_position_int.lon == 0) {
  + position.latitude_deg = double(NAN);
  + position.longitude_deg = double(NAN);
  + } else {
  position.latitude_deg = global_position_int.lat * 1e-7;
  position.longitude_deg = global_position_int.lon * 1e-7;
  + }
  position.absolute_altitude_m = global_position_int.alt * 1e-3f;
  position.relative_altitude_m = global_position_int.relative_alt * 1e-3f;
  set_position(position);
  