xrSound: wait for prefill task before stopping sound playback

Fixes "SOUND: Invalid emitter state" crash.

Author: Xottab-DUTY

src/xrSound/SoundRender_Emitter_StartStop.cpp

@@ -105,6 +105,7 @@ void CSoundRender_Emitter::cancel()
 
 void CSoundRender_Emitter::stop_target()
 {
+    wait_prefill();
     R_ASSERT1_CURE(target, true, { return; });
     target->stop();
     target = nullptr;