Fix "the two" decoder issues

https://www.w3.org/Fonts/WG/track/actions/205 doesn't seem to specify what
"the two" decoder issues are. So I fixed two decoder issues :D (
https://github.com/google/woff2/pull/81).

However, I see a couple more that I suspect we want fixes for before
advancing spec state:

https://github.com/google/woff2/issues/53 Reject loca with mismatching
origLength
https://github.com/google/woff2/issues/54 Reject empty glyph with bounding
box
https://github.com/google/woff2/issues/55 Reject transformed hmtx table
with reserved flags bits set