admin_server: Avoid accessing unallocated memory

In 68b726b93c11cc90 we tried to fix a mem leak. However, it
wasn't done quite well. Problem is, virNetDaemonGetServers() may
fail in which case virObjectListFreeCount() would be called with
-1 objects to free. But the number of elements is taken in
unsigned rather than signed integer.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>

daemon/admin_server.c

@@ -54,6 +54,7 @@ adminConnectListServers(virNetDaemonPtr dmn,
         srvs = NULL;
     }
  cleanup:
+    if (ret > 0)
         virObjectListFreeCount(srvs, ret);
     return ret;
 }