Merge #10151: [logging] initialize flag variable to 0 (and continue if GetLogCategory() fails)

Summary:
cd7f394 initialize flag variable to 0 (and continue if GetLogCategory() fails) (John Newbery)

Tree-SHA512: d0f2653bd0e71ed763220cb08d3a5335c5bdfe2f54ff7f9302d97f3265d7aa7f57606fe416a61aaac1535dbb046d0fb40a61f5a9d5cf234b042268e00ee7679d

Backport of Core PR10151
https://github.com/bitcoin/bitcoin/pull/10151/

Test Plan:

make check
test_runner.py

Reviewers: deadalnix, jasonbcox, Fabien, markblundeberg, O1 Bitcoin ABC, #bitcoin_abc

Reviewed By: markblundeberg, O1 Bitcoin ABC, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D3372