patch 9.0.0416: ml_get error when appending lines in popup window

Problem:    ml_get error when appending lines in popup window.
Solution:   Only update w_topline when w_buffer matches curbuf.
            (closes #11074)
diff --git a/src/evalbuffer.c b/src/evalbuffer.c
index 15ac580..3527737 100644
--- a/src/evalbuffer.c
+++ b/src/evalbuffer.c
@@ -254,7 +254,11 @@
 		    && wp->w_cursor.lnum > append_lnum)
 		wp->w_cursor.lnum += added;
 	check_cursor_col();
-	update_topline();
+
+	// Only update the window view if w_buffer matches curbuf, otherwise
+	// the computations will be wrong.
+	if (curwin->w_buffer == curbuf)
+	    update_topline();
     }
 
 done: