fix g++ warning about assigning NULL to a non-pointer

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@56987 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/msw/printdlg.cpp

@@ -697,7 +697,7 @@ bool wxWindowsPrintDialog::ConvertToNative( wxPrintDialogData &data )
 
     pd->hwndOwner = NULL;
     pd->hInstance = NULL;
-    pd->lCustData = NULL;
+    pd->lCustData = 0;
     pd->lpfnPrintHook = NULL;
     pd->lpfnSetupHook = NULL;
     pd->lpPrintTemplateName = NULL;