stellar-strkey vulnerable to panic in SignedPayload::from_payload

Impact Panic vulnerability when a specially crafted payload is used. This is because of the following calculation: rust innerpayloadlen + 4 - innerpayloadlen % 4 % 4 If innerpayloadlen is 0xffffffff, 4 - innerpayloadlen % 4 % 4 = 1 so rust innerpayloadlen + 4 - innerpayloadlen % 4 % 4 = u32::MAX ...