fix “no per-thread pool” abort

For some backends the process thread can change (e.g.
switch coreaudio headphone + internal speakers)

If there are existing x-thread event calls this can lead to
the following situation:

1) SessionEvent::operator new
2) audioengine process thread change
3) SessionEvent::operator delete  -> crash, wrong thread

SessionEvent::operator delete can safely push the event back to
the pool for later cleanup..

1 files changed, 1 insertions, 1 deletions

diff --git a/libs/pbd/pbd/pool.h b/libs/pbd/pbd/pool.h
index 1c9ac81a3e..cfd782a794 100644
--- a/libs/pbd/pbd/pool.h
+++ b/libs/pbd/pbd/pool.h
@@ -128,7 +128,7 @@ class LIBPBD_API PerThreadPool
         const Glib::Threads::Private<CrossThreadPool>& key() const { return _key; }
 
 	void  create_per_thread_pool (std::string name, unsigned long item_size, unsigned long nitems);
-	CrossThreadPool* per_thread_pool ();
+	CrossThreadPool* per_thread_pool (bool must_exist = true);
 
 	void set_trash (RingBuffer<CrossThreadPool*>* t);
 	void add_to_trash (CrossThreadPool *);