Explicitly convert int to HANDLE to fix 64 bit wxMSW build.

This fixes g++ compilation problem in 64 bit mode after the changes of r76653,
it complained about comparing pointer (HANDLE) with an integer. It might also
fix compilation with icc.

See #16233.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@76658 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775