This bug was introduced in swig-3.0.8 in #146252 adding shared_ptr
support. An ObjectPreviouslyDeleted error was incorrectly thrown
when the pointer was used as a parameter after being set to zero
via a call to 'DATA_PTR(self) = 0'.
It isn't clear to me which approach is better in this corner case,
so I've gone for backwards compatibility and restored the old behaviour.
Closes#602
