Fix assert in PA backend when no sounds are played.
Fix assert in PA backend when no sounds are played.

While here, fix some fresh compiler warnings and a stale debug message.

freedesktop-unofficial-mirror/swfdec__swfdec,mltframework/swfdec,mltframework/swfdec,freedesktop-unofficial-mirror/swfdec__swfdec,freedesktop-unofficial-mirror/swfdec__swfdec