ofp-group: Don't assert-fail decoding bad OF1.5 group mod type or command.

When decoding a group mod, the current code validates the group type and
command after the whole group mod has been decoded.  The OF1.5 decoder,
however, tries to use the type and command earlier, when it might still be
invalid.  This caused an assertion failure (via OVS_NOT_REACHED).  This
commit fixes the problem.

ovs-vswitchd does not enable support for OpenFlow 1.5 by default.

Reported-at: https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=9249
Signed-off-by: Ben Pfaff <blp@ovn.org>
Reviewed-by: Yifeng Sun <pkusunyifeng@gmail.com>
ofputil_make_flow_mod_table_id(bool flow_mod_table_id)
{
    struct nx_flow_mod_table_id *nfmti;
    struct ofpbuf *msg;

    msg = ofpraw_alloc(OFPRAW_NXT_FLOW_MOD_TABLE_ID, OFP10_VERSION, 0);
    nfmti = ofpbuf_put_zeros(msg, sizeof *nfmti);
    nfmti->set = flow_mod_table_id;
    return msg;
}

ofputil_make_flow_mod_table_id(bool flow_mod_table_id)
{
    struct nx_flow_mod_table_id *nfmti;
    struct ofpbuf *msg;

    msg = ofpraw_alloc(OFPRAW_NXT_FLOW_MOD_TABLE_ID, OFP10_VERSION, 0);
    nfmti = ofpbuf_put_zeros(msg, sizeof *nfmti);
    nfmti->set = flow_mod_table_id;
    return msg;
}
