FusionStyle: Don't try to draw null pixmaps

We have the same check for null already for PE_IndicatorHeaderArrow
which actually uses the same pixmap. If the file is not found the
pixmap will be null and the code dividing by its width or height will
thrown an arithmetic exception.

Task-number: QTBUG-43067
Change-Id: I13a5ee9f21f4189b7bbcfd57a6f5b52113de834d
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@theqtcompany.com>
Reviewed-by: J-P Nurmi <jpnurmi@theqtcompany.com>

src/widgets/styles/qfusionstyle.cpp

@@ -514,6 +514,9 @@ void QFusionStyle::drawPrimitive(PrimitiveElement elem,
             break;
         }
         arrow = colorizedImage(QLatin1String(":/qt-project.org/styles/commonstyle/images/fusion_arrow.png"), arrowColor, rotation);
+        if (arrow.isNull())
+            break;
+
         QRect rect = option->rect;
         QRect arrowRect;
         int imageMax = qMin(arrow.height(), arrow.width());