Forego updating sz in capn_write_mem
Forego updating sz in capn_write_mem

It is not used and we already validated the total buffer size
beforehand.

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