Fix compilation of tst_QGraphicsItem in Visual Studio

Visual Studio complained about the usage of a potentially uninitialized
variable which made compilation fail.

Change-Id: I0bc9d1e47d3b00b047912164c3bc4197a2058f85
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@qt.io>
Reviewed-by: Maurice Kalinowski <maurice.kalinowski@qt.io>

tests/auto/widgets/graphicsview/qgraphicsitem/tst_qgraphicsitem.cpp

@@ -477,7 +477,7 @@ private:
 void tst_QGraphicsItem::construction()
 {
     for (int i = 0; i < 7; ++i) {
-        QGraphicsItem *item;
+        QGraphicsItem *item = nullptr;
         switch (i) {
         case 0:
             item = new QGraphicsEllipseItem;