RichTextLabel: clear the scroll offset.

(cherry picked from commit 970dc91cb7)

scene/gui/rich_text_label.cpp

@@ -1453,6 +1453,7 @@ void RichTextLabel::clear() {
 	main->lines.clear();
 	main->lines.resize(1);
 	main->first_invalid_line = 0;
+	scroll_w = 0;
 	update();
 	selection.click = NULL;
 	selection.active = false;