Session::process() returns early with Session::_silent set to true. AudioBuffer::set_data()
was never set for (at least) the LTC output port. PortManager::cycle_end() calls
AudioBuffer::silence() which used to assume that get_buffer() must have been called. But it
was not, because that should have happened in Session::process().
So check AudioBuffer::data() and call get_buffer() if required.
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