AudioServer: Remove any pending mixer queues whose client disconnected

We were pumping the sound device full of silence even after the last
audio client disconnected.

Servers/AudioServer/ASMixer.cpp

@@ -49,6 +49,8 @@ void ASMixer::mix()
             continue;
         }
 
+        active_mix_queues.remove_all_matching([&](auto& entry) { return !entry->client(); });
+
         ASample mixed_buffer[1024];
         auto mixed_buffer_length = (int)(sizeof(mixed_buffer) / sizeof(ASample));