Sample project: reproducible crash

[amyworrall]

Steps to reproduce:

1. Run the sample project
2. Using the '-' button, delete all the pages
3. Add one single new page using the '+' button
4. Attempt to delete that page with the '-' button

There's a crash in pageScrollView:viewForPageAtIndex:, "Terminating app due to uncaught exception 'NSRangeException', reason: '*** -[__NSArrayM objectAtIndex:]: index 4294967295 beyond bounds for empty array'"