[sound applet] Avoid double destroy.

This fixes a bug that shows when running multiple sound-applet compatible media players;
when you stop the second player the following is printed to the console:

	    JS ERROR: !!!   Exception in callback for signal: destroy
	    JS ERROR: !!!     message = '"No signal connection 3 found"'
	    JS ERROR: !!!     lineNumber = '74'
	    JS ERROR: !!!     fileName = '"/usr/share/gjs-1.0/signals.js"'
	    JS ERROR: !!!     stack = '"_disconnect(3)@/usr/share/gjs-1.0/signals.js:74
	([object Object])@/usr/share/cinnamon/js/ui/popupMenu.js:961
	_emit("destroy")@/usr/share/gjs-1.0/signals.js:124
	()@/usr/share/cinnamon/js/ui/popupMenu.js:147
	()@/usr/share/cinnamon/applets/sound@cinnamon.org/applet.js:894
	("org.mpris.MediaPlayer2.audacious")@/usr/share/cinnamon/applets/sound@cinnamon.org/applet.js:864
	("org.mpris.MediaPlayer2.audacious",":1.197")@/usr/share/cinnamon/applets/sound@cinnamon.org/applet.js:882