Fix uninitialized memory read when opening the settings

==25792== Conditional jump or move depends on uninitialised value(s)
==25792==    at 0xFC1F7C2: PreferredScreenSelector::preferredScreen() const (dlgpresentation.cpp:215)
==25792==    by 0xFC1F422: PreferredScreenSelector::repopulateList() (dlgpresentation.cpp:163)
==25792==    by 0xFC1F23F: PreferredScreenSelector::PreferredScreenSelector(QWidget*) (dlgpresentation.cpp:151)
==25792==    by 0xFC1EFBB: DlgPresentation::DlgPresentation(QWidget*) (dlgpresentation.cpp:128)
==25792==    by 0xFC15825: PreferencesDialog::PreferencesDialog(QWidget*, KConfigSkeleton*, Okular::EmbedMode) (preferencesdialog.cpp:45)
==25792==    by 0xFC4A522: Okular::Part::slotPreferences() (part.cpp:2834)