[wayland] fix crash on startup with lv3:ralt_alt XKB option

With lv3:ralt_alt ("Right Alt never chooses 3rd level") option set, we
get more layouts from libxkbcommon than it was configured, see:
xkbcommon/libxkbcommon#262
It might be correct lib's behavior, still.

The extra layouts are redundant, so we strip them out from usual usage.

BUG: 440027

src/xkb.cpp

@@ -413,9 +413,9 @@ QString Xkb::layoutName() const
     return layoutName(m_currentLayout);
 }
 
-const QString &Xkb::layoutShortName(int index) const
+QString Xkb::layoutShortName(int index) const
 {
-    return m_layoutList.at(index);
+    return m_layoutList.value(index);
 }
 
 void Xkb::updateConsumedModifiers(uint32_t key)

src/xkb.h

@@ -89,7 +89,7 @@ class KWIN_EXPORT Xkb : public QObject
     }
     QString layoutName(xkb_layout_index_t index) const;
     QString layoutName() const;
-    const QString &layoutShortName(int index) const;
+    QString layoutShortName(int index) const;
     quint32 numberOfLayouts() const;
 
     /**