Fix a currently unused unique_lock function. Thanks to Lioncash.

Source/Core/Common/Src/StdMutex.h

@@ -318,9 +318,12 @@ public:
 
 	mutex_type* release()
 	{
-		return mutex();
+		auto const ret = mutex();
+
 		pm = NULL;
 		owns = false;
+
+		return ret;
 	}
 
 	bool owns_lock() const