Make sure $pagenow exists before trying to use it

Quick fix [for the bug](https://github.com/Automattic/jetpack/commit/4ac8ceaf30263932175bcf45f6032a48fc3122c4#commitcomment-5855029) referenced by @georgestephanis earlier.
Merge pull request #435 from johnpbloch/patch-1

Make sure $pagenow exists before trying to use it