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/message.c b/src/message.c
index 11662ba..46cf1e0 100644
--- a/src/message.c
+++ b/src/message.c
@@ -1143,7 +1143,7 @@
     FILE	*save_scriptout;
 
     if (redraw == TRUE)
-	must_redraw = UPD_CLEAR;
+	set_must_redraw(UPD_CLEAR);
 
     // If using ":silent cmd", don't wait for a return.  Also don't set
     // need_wait_return to do it later.
@@ -2490,8 +2490,7 @@
     }
 #endif
     ++msg_scrolled;
-    if (must_redraw < UPD_VALID)
-	must_redraw = UPD_VALID;
+    set_must_redraw(UPD_VALID);
 }
 
 /*