message: core: fix bad-cast on param_get fail
  If we did not successfully receive a param we should not try to cast it
  otherwise it will crash with an abort and we'll be quite unhappy.
diff: @@ -679,7 +679,11 @@ void MAVLinkSystem::receive_float_param(bool success, MAVLinkParameters::ParamVa
  get_param_float_callback_t callback)
  {
  if (callback) {
  + if (success) {
  callback(success, value.get_float());
  + } else {
  + callback(success, NAN);
  + }
  }
  }
  @@ -687,7 +691,11 @@ void MAVLinkSystem::receive_int_param(bool success, MAVLinkParameters::ParamValu
  get_param_int_callback_t callback)
  {
  if (callback) {
  + if (success) {
  callback(success, value.get_int32());
  + } else {
  + callback(success, 0);
  + }
  }
  }
  