The Analyser only holds a weak-pointer the the Source,
session-destruction frees the actual Source, which is fatal for any
ongoing audio analysis.
This fix simply waits for the current ongoing analysis to complete,
ideally TransientDetector::run, EBUr128Analysis::run, OnsetDetector::run
and AudioAnalyser::analyse could be interrupted.
Alternate option: cancel the Analyser::work thread (and re-create with
the every session).
71520c7432