wallet: Initialize m_last_block_processed to nullptr. Initialize fields where defined.
wallet: Initialize m_last_block_processed to nullptr. Initialize fields where defined.

Summary: This is a backport of Core PR12785 .

Test Plan:
  make check

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D3634

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