WIP: valgrind runtime analysis and fixes (#6521)

* Fix several valgrind identified mem leaks

* Added callgrind target.

* px4_posix_tasks use nullptr

src/modules/simulator/simulator.h

@@ -278,7 +278,14 @@ private:
 			_actuator_outputs_sub[i] = -1;
 		}
 	}
-	~Simulator() { _instance = NULL; }
+	~Simulator()
+	{
+		if (_instance != nullptr) {
+			delete _instance;
+		}
+
+		_instance = NULL;
+	}
 
 	void initializeSensorData();