message: fix(core): transactionBuilder: ignore `walletSubPath === 'm'`
  The `m` component is only allowed at the very beginning, but some of
  our tests have it in the middle of a path.
  Sadly our buggy `deriveKey` method allows it anywhere (and simply
  ignores it).
  Issue:
diff: @@ -921,8 +921,11 @@ exports.signTransaction = function (params) {
  const chainPath = currentUnspent.chainPath;
  if (rootExtKeyPath) {
  - const subPath = keychain.walletSubPath || '/0/0';
  - const path = keychain.path + subPath + chainPath;
  + let { walletSubPath = '/0/0' } = keychain;
  + if (walletSubPath === 'm') {
  + walletSubPath = ''
  + }
  + const path = keychain.path + walletSubPath + chainPath;
  privKey = rootExtKeyPath.deriveKey(path);
  }
  