Created attachment 4213 
Fixes the crash

Steps to reproduce:
1. Run Ristretto without arguments in a folder without images.
2. Use the menu to hide and then show the thumbnail bar.
3. Crash!

The icon bar size_allocate callback dereferences active_item when none exists.  Simply preventing the crash makes the thumbnail bar appear, but without any images, it has no width and looks bad (it really shouldn't appear to begin with).

The attached patch does a few things to remedy the problem and similar undiscovered problems:
Direct: The menu item only actually shows the thumbnail bar if there are items to display.
Reactive: active_item is only dereferenced in the size_allocate callback if it is not NULL.
Proactive: if size_allocate gets called when active_item is NULL, a g_warning is printed.