refactor AudioEngine-Linux

use std::map::insert properly

Author: JohnCoconut

cocos/audio/linux/AudioEngine-linux.cpp

@@ -320,10 +320,9 @@ int AudioEngineImpl::preload(const std::string& filePath, std::function<void(boo
     }
 
     int id = static_cast<int>(mapChannelInfo.size()) + 1;
-    if (mapId.find(filePath) == mapId.end())
-        mapId.insert({filePath, id});
-    else
-        id = mapId.at(filePath);
+    // std::map::insert returns std::pair<iter, bool>
+    auto pIterBool = mapId.insert({filePath, id});
+    id = pIterBool.first->second;
 
     auto& chanelInfo = mapChannelInfo[id];
     chanelInfo.sound = sound;