Check item index in wxListCtrl::GetItemState() in wxMSW too

This was already the case in the generic version, but wxMSW one just
silently returned 0 for invalid item index.

Make it consistent with the other platforms and SetItemState() by
checking the index in it too.

Closes https://github.com/wxWidgets/wxWidgets/pull/1702

docs/changes.txt

@@ -87,6 +87,9 @@ Changes in behaviour not resulting in compilation errors
   there is no highlighted menu item, instead of wxID_ANY used before, for
   consistency with wxMSW.
 
+- wxListCtrl::GetItemState() in wxMSW now checks the passed in item index for
+  validity, as the generic version under the other platforms already did.
+
 
 Changes in behaviour which may result in build errors
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