Fix a rare EventList race-condition/crash
The GUI thread may modify fade-in/out while the butler-thread
reads audio.
e.g. select a Range and click delete.
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Thread 0:: Dispatch queue: com.apple.main-thread
0 libsystem_pthread.dylib 0x00007fffd45924fc pthread_mutex_lock + 0
1 libglib-2.0.0.dylib 0x00000001085a9d2a g_mutex_lock + 26
2 libevoral.dylib 0x0000000107fd0a49 PBD::Signal0<void, PBD::OptionalLastValue<void> >::operator()() + 57
3 libevoral.dylib 0x0000000107fd486d Evoral::ControlList::clear() + 253
4 libardour.dylib 0x00000001072ef9a5 ARDOUR::AudioRegion::set_fade_out(ARDOUR::FadeShape, long long) + 309
5 libardour.dylib 0x00000001072f19ea ARDOUR::AudioRegion::recompute_at_end() + 122
6 libpbd.dylib 0x00000001082993ff PBD::Stateful::resume_property_changes() + 191
7 libardour.dylib 0x00000001076476af ARDOUR::Playlist::cut(long long, long long, bool) + 575
8 libardour.dylib 0x0000000107646b5b ARDOUR::Playlist::cut_copy(boost::shared_ptr<ARDOUR::Playlist> (ARDOUR::Playlist::*)(long long, long long, bool), std::__1::list<ARDOUR::AudioRange, std::__1::allocator<ARDOUR::AudioRange> >&, bool) + 187
9 libardour.dylib 0x0000000107647461 ARDOUR::Playlist::cut(std::__1::list<ARDOUR::AudioRange, std::__1::allocator<ARDOUR::AudioRange> >&, bool) + 33
10 Ardour.bin 0x00000001065f0fa0 RouteTimeAxisView::cut_copy_clear(Selection&, Editing::CutCopyOp) + 592
11 Ardour.bin 0x0000000106118a94 Editor::cut_copy_ranges(Editing::CutCopyOp) + 164
12 Ardour.bin 0x0000000106116053 Editor::cut_copy(Editing::CutCopyOp) + 1587
Thread 20 Crashed:
0 libardour.dylib 0x00000001072f4b19 ARDOUR::AudioRegion::body_range() const + 89
1 libardour.dylib 0x00000001072bd318 ARDOUR::AudioPlaylist::read(float*, float*, float*, long long, long long, unsigned int) + 1176
2 libardour.dylib 0x00000001072ac236 ARDOUR::AudioDiskstream::read(float*, float*, float*, long long&, long long, int, bool) + 854
3 libardour.dylib 0x00000001072abbb8 ARDOUR::AudioDiskstream::overwrite_existing_buffers() + 392
4 libardour.dylib 0x00000001077ef36a ARDOUR::Session::non_realtime_overwrite(int, bool&) + 186
5 libardour.dylib 0x00000001077ed7f0 ARDOUR::Session::butler_transport_work() + 1696
6 libardour.dylib 0x0000000107323425 ARDOUR::Butler::thread_work() + 149
7 libardour.dylib 0x000000010732334f ARDOUR::Butler::_thread_work(void*) + 95
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