NodeFragment: temporary fix memory leaks

When an error message is allocated on the heap, it can't currently be
freed properly, because nearly every message is allocated at compile
time. There also isn't the possiblity to set a flag, because it can't be
relied, that status is defined on the callee's side. Thus, the only
method to deal with this is by freeing it by the caller, when he knows,
that the message is allocated on the heap.