patch 8.2.1345: Redraw error when using visual block and scroll

Problem:    Redraw error when using visual block and scroll.
Solution:   Add check for w_topline. ( closes #6597)
diff --git a/src/drawscreen.c b/src/drawscreen.c
index faecaa4..7afcf09 100644
--- a/src/drawscreen.c
+++ b/src/drawscreen.c
@@ -1659,10 +1659,13 @@
 #endif
 	    )
     {
-	if (mod_top != 0 && wp->w_topline == mod_top)
+	if (mod_top != 0
+		&& wp->w_topline == mod_top
+		&& (!wp->w_lines[0].wl_valid
+		    || wp->w_topline == wp->w_lines[0].wl_lnum))
 	{
-	    // w_topline is the first changed line, the scrolling will be done
-	    // further down.
+	    // w_topline is the first changed line and window is not scrolled,
+	    // the scrolling from changed lines will be done further down.
 	}
 	else if (wp->w_lines[0].wl_valid
 		&& (wp->w_topline < wp->w_lines[0].wl_lnum