QmlDesigner: crash fix

If the root node is invalid currentState is also invalid.

Change-Id: Icf47c9295cdf82bf7b36328e76dbb1a974ee43f9
Reviewed-on: http://codereview.qt.nokia.com/1706


Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Thomas Hartmann <Thomas.Hartmann@nokia.com>