Fix cmap wrong idRangeOffset

Fixes an issue with possibly malformed cmap tables. Loading e.g. the [TrashHand](http://www.dafont.com/fr/trash-hand.font) font leads to a "RangeError: argument 1 accesses an index that is out of range".

This issue has already been dealt with in the FreeType library :
http://marc.info/?l=freetype&m=91764687315696&w=2