util/Manual: use C++11 initializer

src/util/Manual.hxx

 /*
- * Copyright (C) 2013 Max Kellermann <max.kellermann@gmail.com>
+ * Copyright (C) 2013-2017 Max Kellermann <max.kellermann@gmail.com>
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
@@ -54,12 +54,11 @@ class Manual {
 	char data[sizeof(T)];
 
 #ifndef NDEBUG
-	bool initialized;
+	bool initialized = false;
 #endif
 
 public:
 #ifndef NDEBUG
-	Manual():initialized(false) {}
 	~Manual() {
 		assert(!initialized);
 	}