fix: handle zero-input transaction parsing

[0xsatoshi99]

Summary

Fixes parsing of transactions and PSBTs with zero inputs.

Problem

When parsing a transaction with 0 inputs, the byte sequence 00 01 (vinLen=0, voutLen=1) was incorrectly interpreted as the segwit marker/flag bytes. This caused:

• Transaction.fromBuffer() to fail on zero-input transactions
• Psbt.fromBase64() to fail when deserializing PSBTs with no inputs

Solution

After detecting potential segwit marker/flag (00 01), validate the interpretation by checking:

1. If vinLen would be 0 - a segwit tx with 0 inputs is semantically invalid (no witnesses needed)
2. If the buffer is too small to contain the claimed number of inputs (minimum 41 bytes per input)

If either check fails, rewind and parse as a non-segwit transaction.

Testing

const { Psbt } = require("bitcoinjs-lib");
const psbt = new Psbt();
psbt.addOutput({ address: "1BM1sAcrfV6d4zPKytzziu4McLQDsFC2Qc", value: 100000n });
const base64 = psbt.toBase64();
Psbt.fromBase64(base64); // Now works!

// bitcoin-cli generated PSBT also works
Psbt.fromBase64("cHNidP8BACwCAAAAAAEAoHJOGAkAABl2qRRxekyQdFd6Ba+UJxwysknSmKItmIisAAAAAAAA");

All 2660 existing tests pass.

Fixes #2293

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[0xsatoshi99]

@junderw Can you plz check this PR and let me know your feedback? Thanks.