It is possible that Route::monitoring_state() returns
(MonitoringDisk | MonitorSilence)
This lead to various cases where there were is a direct comparison
(ms == MonitoringDisk). DiskReader::run tests for MonitoringDisk
to check if the buffer needs to be zeroed while locating.
Likewise Route::process_output_buffers() also explicitly tests
for both MonitoringDisk and MonitoringDisk.
The issue was likely introduced in fbe8075117 (although
it may have been possible in earlier version when using hardware
monitoring as well).
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