Assume refcount>0 when copying.

Hopefully it will help compilers understand that destructing a copy
can't bring the counter to 0.

STL_Extension/include/CGAL/Handle_for.h

@@ -130,6 +130,7 @@ public:
     Handle_for(const Handle_for& h)
       : ptr_(h.ptr_)
     {
+	CGAL_assume (ptr_->count > 0);
         ++(ptr_->count);
     }