reset() does nothing if no favicon is set

[adam-nielsen]

If you have no favicon set for a page, and just wish to use FavIconX for transient dynamic feedback, then calling reset() has no effect.

Normally this would return the favicon back to its original image, however if there is no original image then rather than removing the favicon as expected (thus returning it to its original state), the reset() call has no effect.

Probably a bit of an obscure use case (no favicon but wanting dynamic feedback) so perhaps only worth fixing if it's very simple!

Behaviour seen in Firefox 48.0 under Linux.