[HarfBuzz] patch for hb-ot-layout.cc to prevent null-deref

[Jonathan Kew]

It looks to me like the _get_gdef, _get_gsub, and _get_gpos functions in hb-ot-layout.cc need an additional null-check, for the case where the face is actually the dummy _hb_face_nil object; this has no ot_layout, and therefore these functions risk deref'ing a null pointer.

I ran into this in the course of investigating a 'head' table issue (see following mail).

Suggested patch is attached.

JK

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