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

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