[HarfBuzz] NULL dereference in hb_apply_context_t

[Kenichi Ishibashi]

Hi Behdad,

The current revision can crash when you reach arabic_fallback_shape()

In arabic_fallback_shape():
...
OT::hb_apply_context_t c (font, buffer, 1/*global mask*/, NULL);
...

The last argument is NULL, but the constructor of hb_apply_context_t
de-references the last argument.

Looks like the last argument (hb_set_digest_t) doesn't matter in this
function. Patch to fix the crash is attached.

Thanks,
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