Milan Crha authored Oct 10, 2019 and Philip Withnall committed Oct 10, 2019

When the _g_dbus_worker_flush_sync() schedules the 'data' and releases
the worker->write_lock, it is possible for the GDBus worker thread thread
to finish the D-Bus call and acquire the worker->write_lock before
the _g_dbus_worker_flush_sync() re-acquires it in the if (data != NULL) body.
When that happens, the ostream_flush_cb() increases the worker->write_num_messages_flushed
and then releases the worker->write_lock. The write lock is reacquired by
the _g_dbus_worker_flush_sync(), which sees that the while condition is satisfied,
thus it doesn't enter the loop body and immediately clears the data members and
frees the data structure itself. The ostream_flush_cb() is still ongoing, possibly
inside flush_data_list_complete(), where it accesses the FlushData, which can be
in any stage of being freed.

Instead, add an explicit boolean flag indicating when the flush is truly finished.

Closes #1896