telephony-maemo6: fix not cleaning up active calls list on exit

This can cause problem if the adapter is reset in the middle of a call
the indicators won't be update properly.
diff --git a/audio/telephony-maemo6.c b/audio/telephony-maemo6.c
index dfa06d9..bf1e0c4 100644
--- a/audio/telephony-maemo6.c
+++ b/audio/telephony-maemo6.c
@@ -2028,6 +2028,9 @@
 	g_free(last_dialed_number);
 	last_dialed_number = NULL;
 
+	g_slist_free(active_calls);
+	active_calls = NULL;
+
 	g_slist_foreach(calls, (GFunc) csd_call_free, NULL);
 	g_slist_free(calls);
 	calls = NULL;