Initialize threadsNeedCleanUp (#2503)

Signed-off-by: Shameek Ganguly <[email protected]>

src/SimulationRunner.hh

@@ -533,7 +533,7 @@ namespace ignition
       private: std::unique_ptr<msgs::WorldControlState> newWorldControlState;
 
       /// \brief Set if we need to remove systems due to entity removal
-      private: bool threadsNeedCleanUp;
+      private: bool threadsNeedCleanUp{false};
 
       friend class LevelManager;
     };