Copying linked graphic between components is broken

IsLinkedGraphic() is better to use here.

Reproducer steps:
-Insert an image to Impress as a link.
-Copy this image (CTRL+C & CTRL+V) to Writer.
Result: Image is not displayed.

Change-Id: Iaf9b686b5c87caf5a0cc15425f8c5988f857bfe6

svx/source/svdraw/svdograf.cxx

@@ -840,7 +840,7 @@ SdrGrafObj& SdrGrafObj::operator=( const SdrGrafObj& rObj )
     aFilterName = rObj.aFilterName;
     bMirrored = rObj.bMirrored;
 
-    if( rObj.pGraphicLink != NULL)
+    if( rObj.IsLinkedGraphic() )
     {
         SetGraphicLink( aFileName, rObj.aReferer, aFilterName );
     }