Fix crash when a standard bus isn't available

Commit 1f6fa1f3

 added a way of
suspending delivery of messages to standard buses when they connect and
resuming delivery when the main loop starts. As a side-effect, we caused
an attempt to do dispatching even after the connection failed. The D-Bus
library doesn't like that.

Task-number: QTBUG-51299
Change-Id: I0c94a5c2846b48c8aea7ffff143564f7fcede890
Reviewed-by: David Faure <david.faure@kdab.com>