[smooth] Avoid integer overflow.

Reported as

  https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=36243

* src/smooth/ftgrays.c(ADD_LONG, SUB_LONG, MUL_LONG, NEG_LONG)
[STANDALONE_]: Removed, unused.
(ADD_INT) [STANDALONE_]: New macro.
(FT_INTEGRATE): Use ADD_INT.
1 file changed