To work around mysterious bug on OS X.

Fixes https://github.com/harfbuzz/harfbuzz/issues/727

Trying to fix:
https://circleci.com/gh/harfbuzz/harfbuzz/6635

Also part of:
https://github.com/harfbuzz/harfbuzz/issues/630

Return empty face on hb_subset failure instead of null. Plus some minor cleanups for _add_head_and_set_loca_version

HB_GLYPH_FLAG_UNSAFE_TO_BREAK means that the glyph with this flag is somehow affected by the previous logical glyph (the previous index in the buffer if ltr and the next index if the buffer is rtl). If these two glyphs are separated by a break (line or otherwise) then the underlying text should be re-shaped on both sides up to corresponding position in the text of some glyph not marked with this flag.