Remove more clang analyzer warnings.

* src/bdf/bdflib.c (_bdf_readstream), src/truetype/ttgload.c
(TT_Load_Glyph): Remove dead stores.
Fixes for compilation with C++.

* src/autofit/hbshim.c (scripts): Change type to `hb_script_t'.
(af_get_coverage): Updated.
(COVERAGE): Add cast.