Fix missing lock in the chain interface

Summary: This was missed in D7937.

Test Plan:

cmake -GNinja .. -DCMAKE_BUILD_TYPE=Debug
ninja check check-functional

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D7947