Fix a 64-bit arithmetic bug I stumbled upon while investigating our handling of recursive IPComp payloads.  (This code is way old and may go away soon in favor of using sys/lib/libz, but committing anyway as plans aren't finalized yet.)
Fix a 64-bit arithmetic bug I stumbled upon while investigating our
handling of recursive IPComp payloads.  (This code is way old and may
go away soon in favor of using sys/lib/libz, but committing anyway as
plans aren't finalized yet.)

ok deraadt@

orumin/openbsd-efivars,orumin/openbsd-efivars,orumin/openbsd-efivars,orumin/openbsd-efivars