Replace `sprintf` with `snprintf`.

Fixes #1233.

* include/freetype/config/ftstdlib.h (ft_sprintf): Replace with...
(ft_snprintf): This new macro.

* src/autofit/afhints.c (af_print_idx): Add argument to pass the buffer
size.
(af_glyph_hints_dump_points, af_glyph_hints_dump_segments,
af_glyph_hints_dump_edges): Updated.

* src/bdf/bdflib.c (BUFSIZE): New macro.
(bdf_parse_properties_, bdf_parse_start_): Use `ft_snprintf`.

* src/tools/ftrandom/ftrandom.c (do_test): Use `snprintf`.