[qtdbus] Fix potential crash on unregistering a dbus object

Regression from 0b6509fc2f0173136db3fc357f09867d8e36ebd8

Will be merged with that commit in upstream.

Change-Id: I67ae8d1aa4430975d10e280112ace082919d9d80
Merge pull request #12 from special/qdbus-unregister-2

[qtdbus] Fix potential crash on unregistering a dbus object