Sanity-check the dimensions of the icon data in _NET_WM_ICON

It seems that somehow there are window icons with negative sizes, which causes the parsing code to crash as the 'cur' pointer ends up going backward into memory not returned by XGetWindowProperty().

It still feels like there's another issue here, because presumably libwnck doesn't crash on this, and a quick inspection of their code suggests that they don't handle negative values at all.

Closes #37 (closed).