We never dereference `face_id', and some implementations might use a
running number instead of a pointer. Additionally, disallowing
value zero was undocumented.
* src/cache/ftccmap.c (FTC_CMapCache_Lookup), src/cache/ftcmanag.c
(FTC_Manager_LookupFace, FTC_Manager_RemoveFaceID): Remove test for
`face_id'.