pyldb: Avoid use-after-free in msg_diff()

pyldb: Avoid use-after-free in msg_diff()

Make a deep copy of the message elements in msg_diff() so that if either
of the input messages are deallocated early, the result does not refer
to non-existing elements.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=14642
BUG: https://bugzilla.samba.org/show_bug.cgi?id=14836

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