Core/LFG: Fix being unable to re-queue after leaving queue

src/server/game/Handlers/LFGHandler.cpp

@@ -71,7 +71,9 @@ void WorldSession::HandleLfgJoinOpcode(WorldPacket& recvData)
     {
         uint32 dungeon;
         recvData >> dungeon;
-        newDungeons.insert((dungeon & 0x00FFFFFF));        // remove the type from the dungeon entry
+        dungeon &= 0x00FFFFFF;                             // remove the type from the dungeon entry
+        if (dungeon)
+            newDungeons.insert(dungeon);
     }
 
     recvData.read_skip<uint32>();                          // for 0..uint8 (always 3) { uint8 (always 0) }