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)
diff --git a/src/popupmenu.c b/src/popupmenu.c
index 7210420..651f510 100644
--- a/src/popupmenu.c
+++ b/src/popupmenu.c
@@ -865,7 +865,8 @@
 	    ++no_u_sync;
 	    resized = prepare_tagpreview(FALSE, FALSE, use_popup);
 	    --no_u_sync;
-	    --RedrawingDisabled;
+	    if (RedrawingDisabled > 0)
+		--RedrawingDisabled;
 	    g_do_tagpreview = 0;
 
 	    if (curwin->w_p_pvw