Cocoa: Fix systray SVG icons.

Regression caused by f3699510.

Task-number: QTBUG-44686
Change-Id: I546422a67d4da29fac196025b09bddcb45c1b641
Reviewed-by: Timur Pocheptsov <Timur.Pocheptsov@digia.com>

src/plugins/platforms/cocoa/qcocoasystemtrayicon.mm

@@ -234,6 +234,10 @@ void QCocoaSystemTrayIcon::updateIcon(const QIcon &icon)
         }
     }
 
+    // Handle SVG icons, which do not return anything for availableSizes().
+    if (!selectedSize.isValid())
+        selectedSize = icon.actualSize(QSize(maxPixmapHeight, maxPixmapHeight), mode);
+
     QPixmap pixmap = icon.pixmap(selectedSize, mode);
 
     // Draw a low-resolution icon if there is not enough pixels for a retina