[cff] Correctly trace SIDs that contain NULL bytes. We need this to properly trace Multiple Master CFFs, which contain two SIDs that are charstrings. This commit makes FreeType also show the last SID, omitted previously due to a bug. * src/cff/cfftypes.h (CFF_FontRec): Add `string_pool_size' field. * src/cff/cffload.c (cff_index_get_pointers): Add argument to return the pool size. Update all callers. * src/cff/cffobjs.c (cff_face_init) [FT_DEBUG_LEVEL_TRACE]: Directly access `cff->strings' to display the non-default strings.