Check for url before url.match

If we do not check if url is set, we may get an "Uncaught TypeError: Cannot call method 'match' of undefined". I see that error at my remote error logging statistics about 5-10 times a day.

Maybe location.href (which is used in the History plugin) is blocked by some agressive Browser plugin or my implementation is wrong. But I don't think I made something wrong, because the only way I use the History plugin is `History.push('#overlay');` and `History.back();`.