keystore GetKeys(): return result instead of writing to reference

Summary:
Add a missing lock to crypter GetKeys() (see core issue #10905)

Backport of core PR10916

Test Plan:

./test/functional/test_runner.py wallet_dump

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2203