ac_slist: Fix a potential segfault in ac_slist_destroy()
ac_slist: Fix a potential segfault in ac_slist_destroy()

If you passed in NULL as the free function pointer we would crash as we
didn't check if it was NULL.

Signed-off-by: Andrew Clayton <02e0a999c50b1f88df7a8f5a04e1b76b35ea6a88@digital-domain.net>

ac000/libac