patch 9.0.1279: display shows lines scrolled down erroneously

Problem:    Display shows lines scrolled down erroneously. (Yishai Lerner)
Solution:   Do not change "wl_lnum" at index zero. (closes #11938)
diff --git a/src/change.c b/src/change.c
index 1e7bdd6..e5798c5 100644
--- a/src/change.c
+++ b/src/change.c
@@ -611,7 +611,9 @@
 		{
 		    if (wp->w_lines[i].wl_lnum >= lnum)
 		    {
-			if (wp->w_lines[i].wl_lnum < lnume)
+			// Do not change wl_lnum at index zero, it is used to
+			// compare with w_topline.  Invalidate it instead.
+			if (wp->w_lines[i].wl_lnum < lnume || i == 0)
 			{
 			    // line included in change
 			    wp->w_lines[i].wl_valid = FALSE;