Clear MIDI masters properly to avoid replay freezes (#36809)

Clear MIDI masters properly to avoid replay freezes (#36809)

While trying to play a replay I noticed that the replay would freeze
when seeking in some cases. After some debugging, I discovered that two
MIDI renderers had each other as master, which caused an infinite loop
processing MIDI events.

I'm not entirely sure of the sequence of events that leads to this
during replay playback, but I did notice that MIDI render masters are
never set to null. This is in the best case just a memory leak, in the
worst case probably the source of the bug, so... I fixed that.