Fix some memory leaks

Free some objects that were allocated with new but not deleted, and in
one case, do not set a pointer to nullptr before deleting, as this
results in a memory leak due to memory allocated not being freed.

src/frontend/qt_sdl/LocalMP.cpp

@@ -311,8 +311,8 @@ void DeInit()
         MPQueue->detach();
     }
 
-    MPQueue = nullptr;
     delete MPQueue;
+    MPQueue = nullptr;
 }
 
 void SetRecvTimeout(int timeout)