[cff] Integer overflows.

Reported as

  https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=2200
  https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=2210

* src/cff/cf2hints.c (cf2_hintmap_insertHint): Use SUB_INT32 and
ADD_INT32.

* src/cff/cf2intrp.c (cf2_interpT2CharString) <cf2_cmdVMOVETO>: Use
ADD_INT32.
3 files changed