zeroc-ice-36: fix build for gcc8

The build was broken failing on unneccessary memsets.
This issue was fixed upstream in 3.7 and discussed in
https://github.com/zeroc-ice/ice/issues/82

The patch pertaining to the error causing the actual failure still
applies nicely onto the 3.6 version.

Hydra logs of breakage: https://hydra.nixos.org/build/100440955/nixlog/1