CVE-2026-49838: GoBGP confederation validation panics on empty AS_PATH attribute

GoBGP accepts a zero-length AS_PATH during UPDATE decoding and later panics while validating that attribute for a confederation eBGP peer. The vulnerable path is in the BGP UPDATE validator: a malformed UPDATE that should be rejected as a malformed AS_PATH instead reaches an unchecked p.Value[0] access, allowing a configured confederation eBGP peer to trigger a denial of service.