bgpd: fix srv6 memory leak detection

The asan memory leak has been detected:
> Direct leak of 16 byte(s) in 1 object(s) allocated from:
>     #0 0x7f9066dadd28 in __interceptor_calloc (/usr/lib/x86_64-linux-gnu/libasan.so.4+0xded28)
>     #1 0x7f9066779b5d in qcalloc lib/memory.c:105
>     #2 0x556d6ca527c2 in vpn_leak_zebra_vrf_sid_update_per_af bgpd/bgp_mplsvpn.c:389
>     #3 0x556d6ca530e1 in vpn_leak_zebra_vrf_sid_update bgpd/bgp_mplsvpn.c:451
>     #4 0x556d6ca64b3b in vpn_leak_postchange bgpd/bgp_mplsvpn.h:311
>     #5 0x556d6ca64b3b in vpn_leak_postchange_all bgpd/bgp_mplsvpn.c:3751
>     #6 0x556d6cb9f116 in bgp_zebra_process_srv6_locator_chunk bgpd/bgp_zebra.c:3337
>     #7 0x7f906685a6b6 in zclient_read lib/zclient.c:4490
>     #8 0x7f9066826a32 in event_call lib/event.c:2011
>     #9 0x7f906675c444 in frr_run lib/libfrr.c:1217
>     #10 0x556d6c980d52 in main bgpd/bgp_main.c:545
>     #11 0x7f9065784c86 in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x21c86)

Fix this by freeing the previous memory chunk.

Fixes: b72c9e1 ("bgpd: cli for SRv6 SID alloc to redirect to vrf (step4)")
Fixes: 527588a ("bgpd: add support for per-VRF SRv6 SID")

Signed-off-by: Philippe Guibert <philippe.guibert@6wind.com>
(cherry picked from commit eea8a8a)

bgpd/bgp_mplsvpn.c

@@ -388,6 +388,9 @@ void vpn_leak_zebra_vrf_sid_update_per_af(struct bgp *bgp, afi_t afi)
 
 	tovpn_sid_ls = XCALLOC(MTYPE_BGP_SRV6_SID, sizeof(struct in6_addr));
 	*tovpn_sid_ls = *tovpn_sid;
+	if (bgp->vpn_policy[afi].tovpn_zebra_vrf_sid_last_sent)
+		XFREE(MTYPE_BGP_SRV6_SID,
+		      bgp->vpn_policy[afi].tovpn_zebra_vrf_sid_last_sent);
 	bgp->vpn_policy[afi].tovpn_zebra_vrf_sid_last_sent = tovpn_sid_ls;
 }
 
@@ -435,6 +438,8 @@ void vpn_leak_zebra_vrf_sid_update_per_vrf(struct bgp *bgp)
 
 	tovpn_sid_ls = XCALLOC(MTYPE_BGP_SRV6_SID, sizeof(struct in6_addr));
 	*tovpn_sid_ls = *tovpn_sid;
+	if (bgp->tovpn_zebra_vrf_sid_last_sent)
+		XFREE(MTYPE_BGP_SRV6_SID, bgp->tovpn_zebra_vrf_sid_last_sent);
 	bgp->tovpn_zebra_vrf_sid_last_sent = tovpn_sid_ls;
 }
 
@@ -482,6 +487,7 @@ void vpn_leak_zebra_vrf_sid_withdraw_per_af(struct bgp *bgp, afi_t afi)
 		bgp->vrf_id, ZEBRA_SEG6_LOCAL_ACTION_UNSPEC, NULL);
 	XFREE(MTYPE_BGP_SRV6_SID,
 	      bgp->vpn_policy[afi].tovpn_zebra_vrf_sid_last_sent);
+	bgp->vpn_policy[afi].tovpn_zebra_vrf_sid_last_sent = NULL;
 }
 
 /*
@@ -508,6 +514,7 @@ void vpn_leak_zebra_vrf_sid_withdraw_per_vrf(struct bgp *bgp)
 			      bgp->vrf_id, ZEBRA_SEG6_LOCAL_ACTION_UNSPEC,
 			      NULL);
 	XFREE(MTYPE_BGP_SRV6_SID, bgp->tovpn_zebra_vrf_sid_last_sent);
+	bgp->tovpn_zebra_vrf_sid_last_sent = NULL;
 }
 
 /*