[sfnt] Additional guards on the POST table.

Fixes timeout (#1055) analyzed by Ben Wagner, reported as

  https://crbug.com/1194092

* src/sfnt/ttload.c (tt_face_load_post): Check POST format.
* src/sfnt/sfobjs.c (sfnt_load_face): Synthesize the missing unicode
charmap only if the glyph names exist.
* src/psnames/psmodule.c (ps_unicode_value): Short cut ".notdef" and
".null".
4 files changed