bgpd: fix process_queue when un-hiding

bgpd: fix process_queue when un-hiding

bgp_process_queue_init() is not called in bgp_create() when leaving the
BGP instance hidden state because of the following goto:

>	if (hidden) {
>		bgp = bgp_old;
>		goto peer_init;
>	}

Upon reconfiguration of the default instance, the prefixes are never set
into a meta queue by mq_add_handler(). They are never processed for
zebra RIB installation and announcements of update/withdraw.

Do not delete the BGP process_queue when hiding.

Fixes: 4d0e7a49cf ("bgpd: VRF-Lite fix default bgp delete")
Signed-off-by: Louis Scalbert <louis.scalbert@6wind.com>