Clear out log handlers when disposing Debug to avoid crashers.

Clear out log handlers when disposing Debug to avoid crashers.

In cases where the IndividualAggregator is destroyed and instantiated
again (as in some of our tests), we could end up calling the Debug
log handlers after destroying the Debug instance (resulting in
g_static_rec_mutex_lock/unlock calls on an invalid freed mutex,
causing a segfault).

Closes: bgo#664158 - Stale log handler can cause segfault when
re-creating Aggregator in a single run