To reflect its actual status as a member rather than a global value.
g_failed_blocks was previously global: 2862aca
-BEGIN VERIFY SCRIPT-
sed -i 's/g_failed_blocks/m_failed_blocks/g' src/validation.cpp
-END VERIFY SCRIPT-
Backport of Core PR13016
https://github.com/bitcoin/bitcoin/pull/13016/files
Completes T579
Depends on D1972