BUG/MEDIUM: filters: Don't try to init filters for disabled proxies

BUG/MEDIUM: filters: Don't try to init filters for disabled proxies

Configuration is parsed for such proxies but not validated. Concretely, it means
check_config_validity() function does almost nothing for such proxies. Thus, we
must be careful to not initialize filters for disabled proxies because the check
callback function is not called. In fact, to be sure to avoid any trouble,
filters for disabled proxies are released.

This patch fixes a segfault at startup if the SPOE is configured for a disabled
proxy. It must be backported as far as 1.7 (maybe with some adaptations).