Python pidl: avoid segfault with "del obj->attr"

Python pidl: avoid segfault with "del obj->attr"

Deleting an attribute in Python (using the "del" statement) is (at
some stages along a winding path, for C objects) converted into
setting the attribute to NULL. Not None, actual NULL. The way we
handled this NULL was to dereference it. This changes the behaviour to
raising an AttributeError, which is more or less what Python does in
similar situations with builtin objects.

Signed-off-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>