Null cmap accelerator_t is wrong, possibly other accelerators as well

[behdad]

It doesn't have subtable and subtable_uvs pointing to null objects for those.

[behdad]

Okay, pretty much all our accelerator_t objects we refer to from hb_face_t have null-pointer-dereference problems. Not sure how to solve these.

[behdad]

I mean, most of our uses don't get to access these if hb_ot_face_data fails to ensure. Maybe that's it. That there's no easy way out.

[behdad]

Quick checking, cmap, cblc/cbdt, sbix, svg, glyf, hmtx, kern, gdef, post are affected. Not sure what the best resolution is.