Avoid warnings when fetching file icons without a CGContextRef

This can happen when instantiating a QFileIconProvider and no suitable
graphics context is available (e.g., when only using a QQuickView).

The proper, longer (5.2) term solution would be to have the platform theme
return a QIcon with a NSImage icon provider.

Task-number: QTBUG-31908
Change-Id: Ibfdb51e0a7dd2c33761a463f4eb6619f0aca19f0
Reviewed-by: Shawn Rutledge <shawn.rutledge@digia.com>

src/plugins/platforms/cocoa/qcocoatheme.mm

@@ -258,8 +258,9 @@ QPixmap QCocoaTheme::fileIconPixmap(const QFileInfo &fileInfo, const QSizeF &siz
         return QPixmap();
 
     NSRect iconRect = NSMakeRect(0, 0, size.width(), size.height());
+    NSGraphicsContext *gc = [NSGraphicsContext currentContext];
     CGImageRef cgImage = [iconImage CGImageForProposedRect:&iconRect
-                                                   context:[NSGraphicsContext currentContext]
+                                                   context:([gc graphicsPort] ? gc : nil)
                                                      hints:nil];
     QPixmap pixmap = QPixmap::fromImage(qt_mac_toQImage(cgImage));
     return pixmap;