ldb: Work around inconsistent behaviour in PyObject_AsMessageElement()

ldb: Work around inconsistent behaviour in PyObject_AsMessageElement()

PyObject_AsMessageElement() has ‘flags’ and ‘attr_name’ parameters to
set properties of the returned MessageElement, but they apply only
*sometimes*.

‘attr_name’ not being set can result in cryptic and misleading error
messages from various ldb operations.

Changing the function’s behaviour to be more consistent could break
existing code, so we work around the issue instead.

Signed-off-by: Joseph Sutton <josephsutton@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>