Do not destroy C++ entity when destroying python binding object.

    C++ entity might still be in use.

src/entity-py.cc

@@ -60,7 +60,7 @@ namespace dynamicgraph {
 	}
 
 	// Return the pointer as a PyCObject
-	return PyCObject_FromVoidPtr((void*)obj, destroy);
+	return PyCObject_FromVoidPtr((void*)obj, NULL);
       }
 
       /**