network: Unconditionally initialize macmap when stopping virtual network

network: Unconditionally initialize macmap when stopping virtual network

Since we can only ever have one reference to obj->macmap, rather
than only clearing obj->macmap during virNetworkObjUnrefMacMap
(e.g. virtual network from networkShutdownNetwork), let's just
unconditionally clear the obj->macmap to ensure that some future
change that created it's own reference to obj->macmap wouldn't
have that reference disappear if virNetworkObjDispose got called.

Signed-off-by: John Ferlan <jferlan@redhat.com>