message: fix compilation issue with GCC 9
diff: @@ -976,17 +976,17 @@ public:
  output_properties_.register_endpoints(list, id + 1 + decltype(input_properties_)::endpoint_count, length);
  }
  - template<typename> std::enable_if_t<sizeof...(TOutputs) == 0>
  + template<size_t i = sizeof...(TOutputs)> std::enable_if_t<i == 0>
  handle_ex() {
  invoke_function_with_tuple(*obj_, func_ptr_, in_args_);
  }
  - template<typename> std::enable_if_t<sizeof...(TOutputs) == 1>
  + template<size_t i = sizeof...(TOutputs)> std::enable_if_t<i == 1>
  handle_ex() {
  std::get<0>(out_args_) = invoke_function_with_tuple(*obj_, func_ptr_, in_args_);
  }
  - template<typename> std::enable_if_t<sizeof...(TOutputs) >= 2>
  + template<size_t i = sizeof...(TOutputs)> std::enable_if_t<i >= 2>
  handle_ex() {
  out_args_ = invoke_function_with_tuple(*obj_, func_ptr_, in_args_);
  }
  @@ -997,7 +997,7 @@ public:
  (void) output;
  LOG_FIBRE("tuple still at %x and of size %u\r\n", (uintptr_t)&in_args_, sizeof(in_args_));
  LOG_FIBRE("invoke function using %d and %.3f\r\n", std::get<0>(in_args_), std::get<1>(in_args_));
  - handle_ex<void>();
  + handle_ex();
  }
  const char * name_;
  