Check that an error really occurred when getting wxChoice data in wxMSW.

We could wrongly return NULL client data if -1 was stored as client data in
wxChoice because it's the same value as CB_ERR and we always interpreted it as
an error, while it may not be one if GetLastError() doesn't indicate it.

See #13883.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@70414 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775