Fix crash in tab completion

Fixes #1. Patch by Maxdamantus.

nix-repl.cc

@@ -218,6 +218,8 @@ void NixRepl::completePrefix(string prefix)
             // Quietly ignore parse errors.
         } catch (EvalError & e) {
             // Quietly ignore evaluation errors.
+        } catch (UndefinedVarError & e) {
+            // Quietly ignore undefined variable errors.
         }
     }
 }