Desktop styles: fix warnings when parent is not set

Change-Id: I80d3b3e1914439a9c36655c0b7f2e722fb23dc1c
Reviewed-by: Jens Bache-Wiig <jens.bache-wiig@digia.com>

src/styles/Desktop/ComboBoxStyle.qml

@@ -49,8 +49,7 @@ Style {
 
         implicitWidth: 115
         implicitHeight: styleItem.implicitHeight
-        width: parent.width
-        height: parent.height
+        anchors.fill: parent
         StyleItem {
             id: styleItem
 

src/styles/Desktop/MenuBarStyle.qml

@@ -64,7 +64,7 @@ Style {
         width: implicitWidth + pixelMetric("menubaritemspacing")
 
         enabled: menuItem.enabled
-        selected: parent.selected || sunken
-        sunken: parent.sunken
+        selected: (parent && parent.selected) || sunken
+        sunken: parent && parent.sunken
     }
 }