[gtkmm] gtk-demo: Don’t remove_page() with invalid index

[Daniel Boles <dboles src gnome org>]

commit 5584c41069c7294d6a145f627a4b8fbccc33be28
Author: Daniel Boles <dboles src gnome org>
Date:   Fri May 26 16:03:36 2017 +0100

    gtk-demo: Don’t remove_page() with invalid index
    
    The decrement was in the iteration expression, so it happened after the
    loop body. So on the first iteration, this would call remove_page() with
    i == get_n_pages(), which is of course 1 past the maximal allowed index.

 demos/gtk-demo/demowindow.cc |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)
---
diff --git a/demos/gtk-demo/demowindow.cc b/demos/gtk-demo/demowindow.cc
index 6ebc863..b0ef8e1 100644
--- a/demos/gtk-demo/demowindow.cc
+++ b/demos/gtk-demo/demowindow.cc
@@ -433,7 +433,7 @@ void DemoWindow::add_data_tabs(const std::string& filename)
 void DemoWindow::remove_data_tabs()
 {
   // Remove all tabs except Info and Source.
-  for (int i = m_Notebook.get_n_pages(); i > 1; --i)
+  for (int i = m_Notebook.get_n_pages(); i-- > 2;)
   {
     m_Notebook.remove_page(i);
   }