Fix use of wrong offset in buffer
Fix use of wrong offset in buffer

The header of packing is a single byte and nothing was written to the
first byte anyway.

baruch/c-capnproto,baruch/c-capnproto,fluffysquirrels/c-capnproto,opensourcerouting/c-capnproto,opensourcerouting/c-capnproto,fluffysquirrels/c-capnproto