[bdf] Fix up user properties.

Fallout reported as
  https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=40027

* src/bdf/bdflib.c (_bdf_add_property): Cosmetic NULL.
(bdf_create_property): Limit allocations to customary signed
FT_Long and NULL-initialize unused storage.
(bdf_free_font): Do not free unused storage.
1 file changed