daemon: Fix possible use-after-free.

This is essentially a backport of
<https://github.com/NixOS/nix/commit/f52b6c944e90b3e35925122779175705fdc02e12>
by Eelco Dolstra <eelco.dolstra@logicblox.com>.

The use-after-free bug would typically manifest when building with
GCC 5.1.