Duke3D: Minor issue with continuous MusicAndSFX sprites and activated sectors

Originally posted here: https://forums.duke4.net/topic/10912-sound-oddity-when-loading-a-savegame-with-rotating-doors/

For MusicAndSFX sprites that are placed in an operated sector (e.g. a door), which are defined to play a continuous looping sound for the duration of their activation (e.g. grinding gears while the door is opening), there is the potential for that sound to play indefinitely if one saves a game while the sector is in motion.

To reproduce, load the map in the attachment and do the following:

1. Open the door directly in front of you.
2. Save while it is opening, then load that save.
3. Once the door comes to a stop, the sound effect will start playing indefinitely.

I suspect that this started with r3083 (commit 9a335de0) but I wasn't able to pin it down exactly (it's definitely between r3079 and r3084 however).

Test map: looping_doorsound_bug.map