[electrum] fix OldKeyStore.get_pubkey_derivation

Summary: Same issue as D14592, but for the "old" keystore.

Test Plan:
python test_runner.py

Create a wallet from the same seed as the unit test, check that you can receive and send transactions.

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D14593