ofproto-dpif-xlate: fix for group liveness propagation

ofproto-dpif-xlate: fix for group liveness propagation

According to OpenFlow v1.3.5 specification a group is considered live,
if it has at least one live bucket in it.  (6.5 Group Table
Modification Messages: "A group is considered live if a least one of
its buckets is live.")

However, OVS implementation incorrectly returns group as live when no
live bucket is found in group_is_alive() function of
ofproto-dpif-xlate.c.

Instead it should return true only if a live bucket is found (that is
!= NULL).

Signed-off-by: László Sűrű <laszlo.suru@ericsson.com>
Co-authored-by: Jan Scheurich <jan.scheurich@ericsson.com>
Signed-off-by: Jan Scheurich <jan.scheurich@ericsson.com>
Acked-by: Jarno Rajahalme <jarno@ovn.org>