Fix segfault in nix-repl / hydra-eval-jobs

If a "with" attribute set fails to evaluate, we have to make sure its
Env record remains unchanged.  Otherwise, repeated evaluation gives a
segfault:

  nix-repl> :a with 0; { a = x; b = x; }
  Added 2 variables.

  nix-repl> a
  error: value is an integer while an attribute set was expected

  nix-repl> b
  Segmentation fault

0 files changed, 0 insertions, 0 deletions