patch 9.0.1571: RedrawingDisabled not used consistently

Problem:    RedrawingDisabled not used consistently.
Solution:   Avoid RedrawingDisabled going negative.  Set RedrawingDisabled in
            win_split_ins(). (closes #11961)

src/screen.c

@@ -2696,7 +2696,8 @@ give_up:
 #endif
 
     entered = FALSE;
-    --RedrawingDisabled;
+    if (RedrawingDisabled > 0)
+	--RedrawingDisabled;
 
     /*
      * Do not apply autocommands more than 3 times to avoid an endless loop
@@ -4496,7 +4497,7 @@ redrawing(void)
 	return 0;
     else
 #endif
-	return ((!RedrawingDisabled
+	return ((RedrawingDisabled == 0
 #ifdef FEAT_EVAL
 		    || ignore_redraw_flag_for_testing
 #endif