python:netcmd:gpo: fix crash when updating an MOTD GPO

python:netcmd:gpo: fix crash when updating an MOTD GPO

When the policy exists already, there is no exception and the code
tries to use the "data" variable, but it doesn't exist because it was
only defined in the exception handling.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=15774

Signed-off-by: Andreas Hasenack <andreas.hasenack@canonical.com>
Reviewed-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>
Reviewed-by: Jennifer Sutton <jennifersutton@catalyst.net.nz>