[ttLib] fix UnboundLocalError for empty loca/glyph tables (#1680)
[ttLib] fix UnboundLocalError for empty loca/glyph tables (#1680)

* Rename last to pos, and next to nextPos
* make sure nextPos is initialized, to avoid UnboundLocalError on an empty glyf table, partially addressing #684
fonttools/fonttools,googlefonts/fonttools