Do not crash when checking out a branch while not currently
on any branch.

Task-number: QTCREATORBUG-5784
Change-Id: I5d007e91c1778e6f09a5d543990d948bc1427599
Reviewed-on: http://codereview.qt.nokia.com/2958


Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Robert Löhning <robert.loehning@nokia.com>