staticd: Fix NULL pointer dereference

When staticd receives a `ZAPI_SRV6_SID_RELEASED` notification from SRv6
SID Manager, it tries to unset the validity flag of `sid`. But since
the `sid` variable is NULL, we get a NULL pointer dereference.

```
=================================================================
==13815==ERROR: AddressSanitizer: SEGV on unknown address 0x000000000060 (pc 0xc14b813d9eac bp 0xffffcb135a40 sp 0xffffcb135a40 T0)
==13815==The signal is caused by a READ memory access.
==13815==Hint: address points to the zero page.
    #0 0xc14b813d9eac in static_zebra_srv6_sid_notify staticd/static_zebra.c:1172
    #1 0xe44e7aa2c194 in zclient_read lib/zclient.c:4746
    FRRouting#2 0xe44e7a9b69d8 in event_call lib/event.c:1984
    FRRouting#3 0xe44e7a85ac28 in frr_run lib/libfrr.c:1246
    FRRouting#4 0xc14b813ccf98 in main staticd/static_main.c:193
    FRRouting#5 0xe44e7a4773f8 in __libc_start_call_main ../sysdeps/nptl/libc_start_call_main.h:58
    FRRouting#6 0xe44e7a4774c8 in __libc_start_main_impl ../csu/libc-start.c:392
    FRRouting#7 0xc14b813cc92c in _start (/usr/lib/frr/staticd+0x1c92c)

AddressSanitizer can not provide additional info.
SUMMARY: AddressSanitizer: SEGV staticd/static_zebra.c:1172 in static_zebra_srv6_sid_notify
==13815==ABORTING
```

This commit fixes the problem by doing a SID lookup first. If the SID
can't be found, we log an error and return. If the SID is found, we go
ahead and unset the validity flag.

Signed-off-by: Carmine Scarpitta <[email protected]>

staticd/static_zebra.c

@@ -1169,6 +1169,19 @@ static int static_zebra_srv6_sid_notify(ZAPI_CALLBACK_ARGS)
 		DEBUGD(&static_dbg_srv6, "%s: SRv6 SID %pI6 %s: RELEASED", __func__, &sid_addr,
 		       srv6_sid_ctx2str(buf, sizeof(buf), &ctx));
 
+		for (ALL_LIST_ELEMENTS_RO(srv6_sids, node, sid)) {
+			if (IPV6_ADDR_SAME(&sid->addr.prefix, &sid_addr)) {
+				found = true;
+				break;
+			}
+		}
+
+		if (!found || !sid) {
+			zlog_err("SRv6 SID %pI6 %s: not found", &sid_addr,
+				 srv6_sid_ctx2str(buf, sizeof(buf), &ctx));
+			return 0;
+		}
+
 		UNSET_FLAG(sid->flags, STATIC_FLAG_SRV6_SID_VALID);
 
 		break;