wallet: Initialize stop_block to nullptr in ScanForWalletTransactions

Summary:

Previously the argument would be untouched if the first block scan
failed. This makes the behavior predictable, and consistent with the
documentation.

Backport of core PR14957 and the remaining of PR15321 (see D5238).

Depends on D5563.

Test Plan:
With Clang:

cmake -GNinja .. -DCMAKE_BUILD_TYPE=Debug -DENABLE_WERROR=ON
ninja all check check-functional

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D5564