[gtk] range: Fix typo in allocate_trough

[Timm Bäder <baedert src gnome org>]

commit fd20e42932ae289074dd35d667352b8bb02ba225
Author: Timm Bäder <mail baedert org>
Date:   Fri Feb 23 10:59:21 2018 +0100

    range: Fix typo in allocate_trough
    
    lower - upper results in something negative of course.

 gtk/gtkrange.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/gtk/gtkrange.c b/gtk/gtkrange.c
index 981409eb6d..ba37dbee34 100644
--- a/gtk/gtkrange.c
+++ b/gtk/gtkrange.c
@@ -1405,7 +1405,7 @@ gtk_range_allocate_trough (GtkGizmo            *gizmo,
 
       level = CLAMP (priv->fill_level, lower, upper - page_size);
 
-      fill = (level - lower) / (lower - upper - page_size);
+      fill = (level - lower) / (upper - lower - page_size);
 
       if (priv->orientation == GTK_ORIENTATION_HORIZONTAL)
         {