Fixes crash if "delete page" was selected in preview of page that being created

Hi,

While taking gollum for a spin i have encountered a crash:

Steps to reproduce;
1. Select "new" -> "Ok"
2. Select "preview"
3. Select "Delete this Page"
4. crash on "wiki.delete_page(page, { :message => "Destroyed #{name} (#{page.format})" })"

Best Regards