Don't assume a total output map in two places in build.cc
Don't assume a total output map in two places in build.cc

Thanks @regnat for catching one of them. The other follows for many of
the same reasons. I'm find fixing others on a need-to-fix basis,
provided their are no regressions.

NixOS/nix,rycee/nix,NixOS/nix,rycee/nix,rycee/nix,NixOS/nix,rycee/nix,NixOS/nix,NixOS/nix,rycee/nix