[autofit, pshinter] Use `FT_OFFSET`.
This avoids
```
runtime error: applying zero offset to null pointer
```
warnings of clang's undefined behaviour sanitizer.
* src/autofit/afcjk.c (af_cjk_hints_link_segments,
af_cjk_hints_compute_edges, af_cjk_hints_compute_blue_edges,
af_cjk_hint_edges, af_cjk_align_edge_points): Do it.
* src/autofit/afhints.c (af_glyph_hints_align_edge_points,
af_glyph_hints_align_strong_points): Ditto.
* src/autofit/aflatin.c (af_latin_metrics_init_widths,
af_latin_hints_link_segments, af_latin_hints_compute_edges,
af_latin_hints_compute_blue_edges, af_latin_hint_edges): Ditto.
* src/pshinter/pshalgo.c (psh_hint_table_init): Ditto.
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