ignore parsing errors, return empty assignment instead

--- a/src/HOL/SMT/Tools/z3_model.ML	Tue Nov 03 10:36:20 2009 +0100
+++ b/src/HOL/SMT/Tools/z3_model.ML	Tue Nov 03 14:07:38 2009 +0100
@@ -66,8 +66,8 @@
 
 fun read_cex ls =
   explode (cat_lines ls)
-  |> Scan.catch (Scan.finite Symbol.stopper (Scan.error cex))
-  |> fst
+  |> try (fst o Scan.finite Symbol.stopper cex)
+  |> the_default []
 
 
 (* translation into terms *)