VCSBaseEditorParameterWidget::mapSetting cope with 0 pointer

Change-Id: I797488c6ace58a6f927fe3d528131682f1832de1
Reviewed-on: http://codereview.qt-project.org/6235


Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Daniel Teske <daniel.teske@nokia.com>