Fix cast of pointer to int in artprov sample.

This prevented the sample from building when using MinGW-64 and also probably
prevented it from working correctly on other 64-bit systems.

Closes #12453.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@65495 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775