made Nitpick more robust

--- a/src/HOL/Tools/Nitpick/nitpick_hol.ML	Thu Mar 03 08:33:55 2016 +0100
+++ b/src/HOL/Tools/Nitpick/nitpick_hol.ML	Thu Mar 03 17:03:09 2016 +0100
@@ -1421,6 +1421,7 @@
   x |> def_props_for_const thy table |> List.last
     |> normalized_rhs_of |> Option.map (prefix_abs_vars s)
   handle List.Empty => NONE
+       | TERM _ => NONE
 
 fun def_of_const_ext thy (unfold_table, fallback_table) (x as (s, _)) =
   if is_built_in_const x orelse original_name s <> s then