twacube: mark get_initial method as const

* spot/twacube/twacube.hh,spot/twacube/twacube.cc: Here.

spot/twacube/twacube.cc

@@ -81,10 +81,10 @@ namespace spot
     init_ = init;
   }
 
-  unsigned twacube::get_initial()
+  unsigned twacube::get_initial() const
   {
     if (theg_.num_states() == 0)
-      new_state();
+      throw std::runtime_error("automaton has no state at all");
 
     return init_;
   }

spot/twacube/twacube.hh

@@ -148,7 +148,7 @@ namespace spot
     void set_initial(unsigned init);
 
     /// \brief Returns the id of the initial state in the automaton.
-    unsigned get_initial();
+    unsigned get_initial() const;
 
     /// \brief Accessor for a state from its id.
     cstate* state_from_int(unsigned i);