patch 9.0.0455: a few problems with 'splitscroll'

Problem:    A few problems with 'splitscroll'.
Solution:   Fix 'splitscroll' problems. (Luuk van Baal, closes #11117)
diff --git a/src/menu.c b/src/menu.c
index 6a93316..432fbe9 100644
--- a/src/menu.c
+++ b/src/menu.c
@@ -436,6 +436,7 @@
 		--curwin->w_height;
 	    curwin->w_winbar_height = h;
 	}
+	curwin->w_prev_height = curwin->w_height;
     }
 
 theend: