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/main.c b/src/main.c
index c6430aa..f4db631 100644
--- a/src/main.c
+++ b/src/main.c
@@ -686,7 +686,7 @@
 			&& !gui.in_use
 #endif
 					)
-	must_redraw = UPD_CLEAR;
+	set_must_redraw(UPD_CLEAR);
     else
     {
 	screenclear();			// clear screen