[ecash-herald] Patch bug in determining tokenId from SWaP txs

Summary:
T2972

tokenId is only found at stackArray[3] for a signal SLP Atomic Swap, not a payment SLP Atomic Swap.

Correct code logic to accurately reflect spec.

Test Plan:
npm test

npm run sendMsgByBlock 798428 and no error msg

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Subscribers: PiRK

Differential Revision: https://reviews.bitcoinabc.org/D14161