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 <[email protected]>
Co-authored-by: Jan Scheurich <[email protected]>
Signed-off-by: Jan Scheurich <[email protected]>
Acked-by: Jarno Rajahalme <[email protected]>

ofproto/ofproto-dpif-xlate.c

@@ -1502,7 +1502,7 @@ group_is_alive(const struct xlate_ctx *ctx, uint32_t group_id, int depth)
 
         bucket = group_first_live_bucket(ctx, group, depth);
         group_dpif_unref(group);
-        return bucket == NULL;
+        return bucket != NULL;
     }
 
     return false;