ospf6d: Prevent heap-buffer-overflow with unknown type

When parsing a osf6 grace lsa field and we receive an
unknown tlv type, ospf6d was not incrementing the pointer
to get beyond the tlv.  Leaving a situation where ospf6d
would parse the packet incorrectly.

Signed-off-by: Iggy Frankovic <iggy07@gmail.com>

ospf6d/ospf6_gr_helper.c

@@ -176,6 +176,7 @@ static int ospf6_extract_grace_lsa_fields(struct ospf6_lsa *lsa,
 				return OSPF6_FAILURE;
 			break;
 		default:
+			sum += TLV_SIZE(tlvh);
 			if (IS_DEBUG_OSPF6_GR)
 				zlog_debug("%s, Ignoring unknown TLV type:%d",
 					   __func__, ntohs(tlvh->type));