determinize: do not work on deterministic automata

* spot/twaalgos/determinize.cc: Here.

spot/twaalgos/determinize.cc

@@ -611,6 +611,9 @@ namespace spot
 		   bool pretty_print, bool use_scc,
 		   bool use_simulation, bool use_stutter)
   {
+    if (a->prop_deterministic())
+      return std::const_pointer_cast<twa_graph>(a);
+
     // Degeneralize
     twa_graph_ptr aut = spot::degeneralize_tba(a);
     std::map<int, bdd> implications;