Fix drawing boxselection on GraphEdit

(cherry picked from commit 1bdc14acea6b2ce932e6cf01dc9d7942f72503e9)

scene/gui/graph_edit.cpp

@@ -1341,6 +1341,7 @@ void GraphEdit::_gui_input(const Ref<InputEvent> &p_ev) {
 
 		if (b->get_button_index() == BUTTON_LEFT && !b->is_pressed() && box_selecting) {
 			box_selecting = false;
+			box_selecting_rect = Rect2();
 			previus_selected.clear();
 			top_layer->update();
 			minimap->update();