ofp-util: Fix use-after-free in group append.

It is possible for ofpbuf_put() to realloc a newly allocated address,
casuing the previously referenced pointer, ogds, points to old/free'd
address. The issue is generated by forcing ofpbuf_put() to use newly
allocated buffer and valgrind reports invalid write. The similiar syndrome
is reported at: https://patchwork.ozlabs.org/patch/591330/

Invalid write of size 2
    ofputil_append_ofp15_group_desc_reply (ofp-util.c:8367)
    ofputil_append_group_desc_reply (ofp-util.c:8392)
    append_group_desc (ofproto.c:6262)
    handle_group_request (ofproto.c:6230)
    handle_group_desc_stats_request (ofproto.c:6269)
    handle_openflow__ (ofproto.c:7337)
    handle_openflow (ofproto.c:7403)
    ofconn_run (connmgr.c:1379)
    connmgr_run (connmgr.c:323)
    ofproto_run (ofproto.c:1762)
    bridge_run__ (bridge.c:2885)
    bridge_run (bridge.c:2940)
    main (ovs-vswitchd.c:120)

Address 0x7cb1020 is 144 bytes inside a block of size 1,144 free'd
    free (vg_replace_malloc.c:530)
    ofpbuf_resize__ (ofpbuf.c:246)
    ofpbuf_put (ofpbuf.c:386)
    nx_put_header__ (nx-match.c:1241)
    nxm_put__ (nx-match.c:697)
    oxm_put_field_array (nx-match.c:1226)
    ofputil_put_group_prop_ntr_selection_method (ofp-util.c:8305)
    ofputil_append_ofp15_group_desc_reply (ofp-util.c:8364)
    ofputil_append_group_desc_reply (ofp-util.c:8392)
    append_group_desc (ofproto.c:6262)

Signed-off-by: William Tu <u9012063@gmail.com>
Signed-off-by: Joe Stringer <joe@ovn.org>

lib/ofp-util.c

@@ -8364,6 +8364,7 @@ ofputil_append_ofp15_group_desc_reply(const struct ofputil_group_desc *gds,
         ofputil_put_group_prop_ntr_selection_method(version, &gds->props,
                                                     reply);
     }
+    ogds = ofpbuf_at_assert(reply, start_ogds, sizeof *ogds);
     ogds->length = htons(reply->size - start_ogds);
 
     ofpmp_postappend(replies, start_ogds);