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.

visual_script.h

@@ -495,7 +495,7 @@ class VisualScriptLanguage : public ScriptLanguage {
 	String _debug_error;
 	int _debug_call_stack_pos = 0;
 	int _debug_max_call_stack;
-	CallLevel *_call_stack;
+	CallLevel *_call_stack = nullptr;
 
 public:
 	StringName notification = "_notification";