[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
+const QString Xkb::layoutShortName(int index) const
 {
-    return m_layoutList.at(index);
+    return m_layoutList.value(index);
 }
 
 void Xkb::updateConsumedModifiers(uint32_t key)
@@ -527,9 +527,7 @@ void Xkb::switchToNextLayout()
     if (!m_keymap || !m_state) {
         return;
     }
-    const xkb_layout_index_t numLayouts = xkb_keymap_num_layouts(m_keymap);
-    const xkb_layout_index_t nextLayout = (xkb_state_serialize_layout(m_state, XKB_STATE_LAYOUT_EFFECTIVE) + 1) % numLayouts;
-    switchToLayout(nextLayout);
+    switchToLayout( (xkb_state_serialize_layout(m_state, XKB_STATE_LAYOUT_EFFECTIVE) + 1) % numberOfLayouts() );
 }
 
 void Xkb::switchToPreviousLayout()
@@ -560,7 +558,7 @@ quint32 Xkb::numberOfLayouts() const
     if (!m_keymap) {
         return 0;
     }
-    return xkb_keymap_num_layouts(m_keymap);
+    return qMin( xkb_keymap_num_layouts(m_keymap), uint(m_layoutList.size()) );
 }
 
 void Xkb::setSeat(KWaylandServer::SeatInterface *seat)

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;
+    const QString layoutShortName(int index) const;
     quint32 numberOfLayouts() const;
 
     /**