fix: rare audio crash during shutdown

[aet]

I'm experiencing a rare audio related crash, maybe once a month. Either in the player or the editor when the whole application is being shut down.

I do have some other audio changes on plugin side that made the issue less likely, but right now it feels like this C# side code ordering change should be good to apply regardless.

=================================================================
    Native Crash Reporting
=================================================================
Got a UNKNOWN while executing native code. This usually indicates
a fatal error in the mono runtime or one of the native libraries 
used by your application.
=================================================================

=================================================================
    Managed Stacktrace:
=================================================================
      at <unknown> <0xffffffff>
      at Unity.WebRTC.NativeMethods:AudioTrackSinkProcessAudio <0x00131>
      at AudioStreamRenderer:SetData <0x000ba>
      at <Module>:invoke_void_single[]_int_int <0x00188>
      at Unity.WebRTC.AudioCustomFilter:OnAudioFilterRead <0x000cb>
      at <Module>:runtime_invoke_void__this___object_int <0x00109>
=================================================================
Crash!!!

[karasusan]

@aet It looks good to me.