FreeTDM: Don't ftdm_safe_free() span information in ftdm_span_destroy() if span provides a custom span_destroy callback.

Doing so will cause a NULL-deref segfault while removing the current span from the
global hash (cur_span->name == NULL) in ftdm_global_destroy().

Remove all the ftdm_safe_free() calls in the custom callback part of ftdm_span_destroy() and
let ftdm_global_destroy() handle that.

NOTE: Also adds a missing ftdm_safe_free(cur_span->dtmf_hangup) to ftdm_global_destroy().
Signed-off-by: Stefan Knoblich <stkn@openisdn.net>