This fixes a crash when deleting routes, while there are still
automation events queued for the route.
Specifically, SoloControl has a reference Soloable& _soloable; which
points to the parent route. A rt-event can still hold a valid shared
pointer to the SoloControl, even if the route is destroyed.
Calling SoloControl::actually_set_value is fine (the control still
exists due to the shared ptr), but then checking the parent route:
```
if (_soloable.is_safe() || !can_solo())
```
accesses the already deleted route, which causes a crash.
The solution implemented here is to not bind a shared_ptr to the
realtime event. However, since deletion of the route happens in the main
UI thread, there may or may not still be a race.
3b9a253a84