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.

scene/gui/spin_box.h

@@ -38,11 +38,11 @@
 class SpinBox : public Range {
 	GDCLASS(SpinBox, Range);
 
-	LineEdit *line_edit;
+	LineEdit *line_edit = nullptr;
 	int last_w = 0;
 	bool update_on_text_changed = false;
 
-	Timer *range_click_timer;
+	Timer *range_click_timer = nullptr;
 	void _range_click_timeout();
 	void _release_mouse();