Qt: fix what seems to be a copy/paste error in MainWindow::loadWindowGeometry() (Coverity CID 1158563)

Change-Id: Iaa2a6367fbb434514751e0ed4f273f883798ce8a
Reviewed-on: https://code.wireshark.org/review/5066
Reviewed-by: Alexis La Goutte <alexis.lagoutte@gmail.com>
Reviewed-by: Anders Broman <a.broman58@gmail.com>