gridview: Fix condition for adding filler tiles

Code above ensures that i is always in [0, n_columns - 1] range, so
the condition was always true, which resulted in filler tile always
being added to the grid. As the result, an empty row appeared at the
end of the grid if the number of columns divided the number of items.

Only add filler tile if last row is not full, i.e. when i > 0.

gtk/gtkgridview.c

@@ -820,7 +820,7 @@ gtk_grid_view_size_allocate (GtkWidget *widget,
         }
     }
   /* Add a filler tile for empty space in the bottom right */
-  if (i < self->n_columns)
+  if (i > 0)
     {
       GtkListTile *filler;
       tile = gtk_list_item_manager_get_last (self->item_manager);