Fix assertion failure in nix-store --export
Fix assertion failure in nix-store --export

Fixes #1173.

shlevy/nix,dtzWill/nix,zimbatm/nix,rycee/nix,vcunat/nix,zimbatm/nix,rycee/nix,ehmry/nix,NixOS/nix,ehmry/nix,zimbatm/nix,rycee/nix,rycee/nix,peti/nix,bennofs/nix,dezgeg/nix,vcunat/nix,layus/nix,vcunat/nix,layus/nix,ehmry/nix,shlevy/nix,dtzWill/nix,pikajude/nix,rycee/nix,pikajude/nix,dezgeg/nix,ehmry/nix,layus/nix,NixOS/nix,peti/nix,ehmry/nix,vcunat/nix,NixOS/nix,layus/nix,bennofs/nix,bennofs/nix,dtzWill/nix,bennofs/nix,peti/nix,dezgeg/nix,peti/nix,shlevy/nix,dtzWill/nix,dezgeg/nix,dezgeg/nix,shlevy/nix,NixOS/nix,layus/nix,peti/nix,shlevy/nix,dtzWill/nix,ehmry/nix,pikajude/nix,zimbatm/nix,NixOS/nix