Do not return stuff on unserialize

This fixes #217.

I did some regex searches in our code base and could not find a place where `unserialize` is called and expected to have a return value (I tried to search for stuff like `... = ...->unserialize( ... )` as well as `...( ...->unserialize( ... )`).

Also see https://github.com/wmde/Diff/pull/15.