[truetype] Some fixes for VF checks.

Reported as

  https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=10317

* src/truetype/ttgxvar.c (ft_var_load_gvar): Properly exit memory
frame if we have invalid glyph variation data offsets.
(tt_face_vary_cvt): Protect against missing `tuplecoords' array.
Fix typo.
2 files changed