Fix possible thread issue.

dll/steam_networking.h

@@ -70,6 +70,8 @@ public ISteamNetworking
 
     std::recursive_mutex messages_mutex;
     std::vector<Common_Message> messages;
+
+    std::recursive_mutex connections_edit_mutex;
     std::vector<struct Steam_Networking_Connection> connections;
 
     std::vector<struct steam_listen_socket> listen_sockets;
@@ -79,11 +81,13 @@ public ISteamNetworking
 
 bool connection_exists(CSteamID id)
 {
+    std::lock_guard<std::recursive_mutex> lock(connections_edit_mutex);
     return std::find_if(connections.begin(), connections.end(), [&id](struct Steam_Networking_Connection const& conn) { return conn.remote == id;}) != connections.end();
 }
 
 struct Steam_Networking_Connection *get_or_create_connection(CSteamID id)
 {
+    std::lock_guard<std::recursive_mutex> lock(connections_edit_mutex);
     auto conn = std::find_if(connections.begin(), connections.end(), [&id](struct Steam_Networking_Connection const& conn) { return conn.remote == id;});
 
     if (connections.end() == conn) {
@@ -98,6 +102,8 @@ struct Steam_Networking_Connection *get_or_create_connection(CSteamID id)
 
 void remove_connection(CSteamID id)
 {
+    {
+        std::lock_guard<std::recursive_mutex> lock(connections_edit_mutex);
         auto conn = std::begin(connections);
         while (conn != std::end(connections)) {
             if (conn->remote == id) {
@@ -107,6 +113,7 @@ void remove_connection(CSteamID id)
                 ++conn;
             }
         }
+    }
 
     //pretty sure steam also clears the entire queue of messages for that connection
     {