Cancel the correct account sync in SyncDaemon::cancel()

When an API to sync per-source was introduced in commit ec16c7a1 the SyncDaemon::cancel() method was incorrectly adapted. It can either be passed a specific account or null in order to sync jobs of all accounts. That is the case when the cancelAll DBus method is called which leads to a null pointer dereference.

This means effectively that each time the SyncMonitor.cancel() is called from QML in reponse to pressing the sync button sync-monitor crashes.

This fixes #52 (closed).