isisd: Fix CID 1568129 (Null pointer dereference)

isisd: Fix CID 1568129 (Null pointer dereference)

Fix this coverity issue:

*** CID 1568129:  Null pointer dereferences  (REVERSE_INULL)
/isisd/isis_tlvs.c: 2813 in unpack_item_srv6_end_sid()
2807                    sid->subsubtlvs = NULL;
2808            }
2809
2810            append_item(&subtlvs->srv6_end_sids, (struct isis_item *)sid);
2811            return 0;
2812     out:
>>>     CID 1568129:  Null pointer dereferences  (REVERSE_INULL)
>>>     Null-checking "sid" suggests that it may be null, but it has already been dereferenced on all paths leading to the check.
2813            if (sid)
2814                    free_item_srv6_end_sid((struct isis_item *)sid);
2815            return 1;
2816     }
2817
2818     /* Functions related to TLVs 1 Area Addresses */

Signed-off-by: Carmine Scarpitta <cscarpit@cisco.com>