Fix bogus error report when disconnect/stop actually works

The interfaces serviceActive does not mean the network is connected, so
we get the next best thing - the interfaces associated ssid will be
empty if a previously connected AP is now disconnected.

Task-number: QTBUG-42087
Change-Id: I539811d9f18cc553a4022a03686fb8a864b98491
Reviewed-by: Alex Blasche <alexander.blasche@digia.com>