Clear roadmap before destroying it

  Data are stored in the constraint graph. Clearing the roadmap also
  clears these data and make problem resolution re-entrant.

src/problem-solver.cc

@@ -312,6 +312,8 @@ namespace hpp {
     {
       if (!problem ())
         throw std::runtime_error ("The problem is not defined.");
+      if (roadmap())
+        roadmap()->clear();
       RoadmapPtr_t r (Roadmap::create (problem ()->distance (), problem ()->robot ()));
       if (constraintGraph_) r->constraintGraph (constraintGraph_);
       roadmap (r);