[wallet] Fix potential memory leak in CreateWalletFromFile

Summary:

Fix proposed by ryanofsky in
https://github.com/bitcoin/bitcoin/pull/12647#discussion_r174875670

This cleans the wallet_hd.py with asan.

Partial backport of core PR10740 (commit 59b87a2)
https://github.com/bitcoin/bitcoin/pull/10740/commits/59b87a27efea819e433c727756bf5fac57b33dd6

Progress towards T459
Depends on D3618

Test Plan:

make check

With ASAN enabled:

./test/functional/test_runner.py wallet_hd

Reviewers: #bitcoin_abc, deadalnix, markblundeberg

Reviewed By: #bitcoin_abc, markblundeberg

Differential Revision: https://reviews.bitcoinabc.org/D3629