Zero initialize all pointer class and struct members

This prevents the pitfall of UB when checking if they have been
assigned something valid by comparing to nullptr.

core/object/message_queue.h

@@ -62,10 +62,10 @@ class MessageQueue {
 		};
 	};
 
-	uint8_t *buffer;
+	uint8_t *buffer = nullptr;
 	uint32_t buffer_end = 0;
 	uint32_t buffer_max_used = 0;
-	uint32_t buffer_size;
+	uint32_t buffer_size = 0;
 
 	void _call_function(const Callable &p_callable, const Variant *p_args, int p_argcount, bool p_show_error);