admin_server: Avoid accessing unallocated memory

In 68b726b9 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: NMichal Privoznik <mprivozn@redhat.com>

daemon/admin_server.c

@@ -54,7 +54,8 @@ adminConnectListServers(virNetDaemonPtr dmn,
         srvs = NULL;
     }
  cleanup:
-    virObjectListFreeCount(srvs, ret);
+    if (ret > 0)
+        virObjectListFreeCount(srvs, ret);
     return ret;
 }