ico: Avoid possible ImageScore overflow

Seeing as this is only a comparison, don't try to do arithmetics that
could overflow.

Detected by UBSan:
io-ico.c:204:9: runtime error: signed integer overflow: 2134193699 -
-555819298 cannot be represented in type 'int'

https://bugzilla.gnome.org/show_bug.cgi?id=776040