autotest: very rare failure on test-block-cache-write

[rouault]

What is the bug?

Happened on build-windows-conda CI config:

FAILED: autotest/cpp/CMakeFiles/test-block-cache-write D:/a/gdal/gdal/build/autotest/cpp/CMakeFiles/test-block-cache-write 
autotest\cpp\CMakeFiles\test-block-cache-write-ef7056b.bat 34f573950bb64298
[==========] Running 1 test from 1 test suite.
[----------] Global test environment set-up.
[----------] 1 test from testblockcachelimits
[ RUN      ] testblockcachelimits.test
Start test1
main thread 00000000000004B0
begin thread
Entering IWriteBlock(0, 0)
Leaving IWriteBlock(0, 0)
end of thread

Entering IWriteBlock(1, 0)

Leaving IWriteBlock(1, 0)

End test1

Start test2

main thread 00000000000004B0

begin thread 00000000000021FC

0

D:/a/gdal/gdal/autotest/cpp/testblockcachewrite.cpp(144): error: Expected equality of these values:

  c

    Which is: '\0'

  1

Steps to reproduce the issue

Versions and provenance

GDAL master as of c8b3d5740d5cf0402dc3c8b22531133eba7c6171 but definitely an issue that must be much more older

Additional context

No response