message: telemetry: add check for invalid, cast to double
diff: @@ -589,7 +589,9 @@ void TelemetryImpl::process_global_position_int(const mavlink_message_t& message
  {
  Telemetry::Heading heading;
  - heading.heading_deg = global_position_int.hdg * 1e-2f;
  + heading.heading_deg = (global_position_int.hdg != UINT16_MAX) ?
  + static_cast<double>(global_position_int.hdg) * 1e-2 :
  + static_cast<double>(NAN);
  set_heading(heading);
  }
  