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

1 files changed, 3 insertions, 3 deletions

diff --git a/src/libexpr/eval.cc b/src/libexpr/eval.cc
index af2bf92e3..050991a1b 100644
--- a/src/libexpr/eval.cc
+++ b/src/libexpr/eval.cc
@@ -319,9 +319,9 @@ inline Value * EvalState::lookupVar(Env * env, const VarRef & var, bool noEval)
     while (1) {
         if (!env->haveWithAttrs) {
             if (noEval) return 0;
-            Expr * attrs = (Expr *) env->values[0];
-            env->values[0] = allocValue();
-            evalAttrs(*env->up, attrs, *env->values[0]);
+            Value * v = allocValue();
+            evalAttrs(*env->up, (Expr *) env->values[0], *v);
+            env->values[0] = v;
             env->haveWithAttrs = true;
         }
         Bindings::iterator j = env->values[0]->attrs->find(var.name);