The convolver may be re-activated shortly after initialization
(e.g. session load, switching snapshots, or buffer-size.
In this case not all process threads may have started.
Convproc::stop_process() skips them (their state is still ST_IDLE).
Yet some short time later the thread's main function runs
and changes the state to ST_PROC, and check_stop () waits forever.
This is solved by waiting for all threads to start.
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