patch 9.0.0505: various problems with 'nosplitscroll'

Problem:    Various problems with 'nosplitscroll'.
Solution:   Fix 'nosplitscroll' problems. (Luuk van Baal, closes #11166)
diff --git a/src/move.c b/src/move.c
index 2ef7a9c..a51d2d1 100644
--- a/src/move.c
+++ b/src/move.c
@@ -991,7 +991,7 @@
     /*
      * First make sure that w_topline is valid (after moving the cursor).
      */
-    if (p_spsc)
+    if (!skip_update_topline)
 	update_topline();
 
     /*