ofproto-dpif: Fix memory leak in disable_datapath_clone()

In testcase "ofproto-dpif - clone action", valgrind reports a memory leak
with the following call stack.
    xrealloc (util.c:123)
    ds_reserve (dynamic-string.c:63)
    ds_put_format_valist (dynamic-string.c:161)
    ds_put_format (dynamic-string.c:142)
    disable_datapath_clone (ofproto-dpif.c:5206)
    process_command (unixctl.c:313)
    run_connection (unixctl.c:347)
    unixctl_server_run (unixctl.c:400)
    main (ovs-vswitchd.c:112)

Signed-off-by: Yi-Hung Wei <yihung.wei@gmail.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>

ofproto/ofproto-dpif.c

@@ -5257,6 +5257,7 @@ disable_datapath_clone(struct unixctl_conn *conn OVS_UNUSED,
     udpif_flush(ofproto->backer->udpif);
     ds_put_format(&ds, "Datapath clone action disabled for bridge %s", br);
     unixctl_command_reply(conn, ds_cstr(&ds));
+    ds_destroy(&ds);
 }
 
 static void