avmedia: fix varargs call for WNT 64-bit

SfxDispatcher::Execute(sal_uInt16 nSlot, SfxCallMode eCall,
    const SfxPoolItem*  pArg1, ...)
assigns the varargs to a "const SfxPoolItem *", so passing in
"0L" is only correct if long is 64-bit, but on WNT 64-bit long is
32-bit, so use "nullptr" already.

Change-Id: Iaf41efa795fd4439daab7ed7ec334d8ad8c476f3

avmedia/source/framework/mediaplayer.cxx

@@ -136,7 +136,7 @@ void MediaFloater::dispatchCurrentURL()
             url = mpMediaWindow->getURL();
         }
         const SfxStringItem aMediaURLItem( SID_INSERT_AVMEDIA, url );
-        pDispatcher->Execute( SID_INSERT_AVMEDIA, SfxCallMode::RECORD, &aMediaURLItem, 0L );
+        pDispatcher->Execute(SID_INSERT_AVMEDIA, SfxCallMode::RECORD, &aMediaURLItem, nullptr);
     }
 }