[fix] reject empty identities when joining transmissions

src/signal_negotiation.cpp

@@ -391,7 +391,7 @@ bool SignalNegotiation::join_transmission(websocketpp::connection_hdl hdl,
   }
 
   std::string user_id;
-  if (!GetStringField(j, "user_id", user_id)) {
+  if (!GetStringField(j, "user_id", user_id) || user_id.empty()) {
     LOG_ERROR("join_transmission missing or invalid field: user_id");
     return false;
   }

src/transmission_manager.cpp

@@ -252,6 +252,9 @@ bool TransmissionManager::BindHostToTransmission(
 bool TransmissionManager::BindGuestToTransmission(
     const std::string& guest_id, const std::string& transmission_id) {
   StateLock lock(*this);
+  if (guest_id.empty() || transmission_id.empty()) {
+    return false;
+  }
   auto host_it = transmission_host_id_list_.find(transmission_id);
   if (host_it == transmission_host_id_list_.end()) {
     LOG_WARN("Transmission [{}] does not exist", transmission_id);