Initialize Unicode code when generating wxEVT_CHAR_HOOK in wxMSW.

Use the fixed value of 0 for the keys without Unicode equivalent instead of
whatever junk was there in the uninitialized variable.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@73719 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775