Fix missing lock in the chain interface
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

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