qt4(Win32): correct crash when saving preferences

The way options are stored in a list by append and retrieve by named index
(enum list) is kind of risky. Errors can easily occur, some are obvious
 (crash), others may be more silent.

modules/gui/qt4/components/simple_preferences.cpp

@@ -519,6 +519,7 @@ SPrefsPanel::SPrefsPanel( intf_thread_t *_p_intf, QWidget *_parent,
             optionWidgets.append( ui.stylesCombo );
 #else
             ui.stylesCombo->hide();
+            optionWidgets.append( NULL );
 #endif
 
             ui.skins_zone->setEnabled( ui.skins->isChecked() );
@@ -760,6 +761,7 @@ void SPrefsPanel::apply()
             config_PutPsz( p_intf, "intf", "skins2" );
         if( qobject_cast<QRadioButton *>(optionWidgets[qtRB])->isChecked() )
             config_PutPsz( p_intf, "intf", "qt" );
+        if( qobject_cast<QComboBox *>(optionWidgets[styleCB]) )
             getSettings()->setValue( "MainWindow/QtStyle",
                 qobject_cast<QComboBox *>(optionWidgets[styleCB])->currentText() );