decoder: terminate decoder thread before MPD cleanup

When MPD exits, it should manually free all resources in use, to allow
easy memory leak debugging.  Make the decoder thread terminate during
that.

src/player_thread.c

@@ -514,6 +514,7 @@ static void * player_task(G_GNUC_UNUSED void *arg)
 			break;
 
 		case PLAYER_COMMAND_EXIT:
+			dc_quit(&pc.notify);
 			closeAudioDevice();
 			player_command_finished();
 			pthread_exit(NULL);