patch 9.0.0245: mechanism to prevent recursive screen updating is incomplete

Problem:    Mechanism to prevent recursive screen updating is incomplete.
Solution:   Add "redraw_not_allowed" and set it in build_stl_str_hl().
            (issue #10952)
diff --git a/src/ex_docmd.c b/src/ex_docmd.c
index 902fc2c..382029a 100644
--- a/src/ex_docmd.c
+++ b/src/ex_docmd.c
@@ -7116,7 +7116,7 @@
 #ifdef FEAT_GUI
 		hold_gui_events = 0;
 #endif
-		must_redraw = UPD_CLEAR;
+		set_must_redraw(UPD_CLEAR);
 		pending_exmode_active = TRUE;
 
 		main_loop(FALSE, TRUE);