Fix ticket unknown-horizons#494.

Unsafe to modify lists you are looping in. You can recreate the bug if u have a large number of emitters. My example used to remove 2 out of 4 ..

git-svn-id: http://svn.unknown-horizons.org/trunk@3248 d3468d7a-13f2-4ea4-bf00-366cb6352a40

horizons/session.py

@@ -125,7 +125,7 @@ def end(self):
 		ExtScheduler().rem_all_classinst_calls(self)
 
 		if Settings().sound.enabled:
-			for emitter in horizons.main.fife.emitter['ambient']:
+			for emitter in horizons.main.fife.emitter['ambient'][:]:
 				emitter.stop()
 				horizons.main.fife.emitter['ambient'].remove(emitter)
 			horizons.main.fife.emitter['effects'].stop()