Avoid spurious error messages for WM_MDISETMENU under Windows XP.

WM_MDISETMENU handler doesn't seem to reset the last error under Windows XP
and this could result in spurious debug error messages when setting the
initial menu in which case NULL is returned to indicate that there was no
previous menu, but this doesn't indicate that an error occurred.

Explicitly reset the last error to ERROR_SUCCESS ourselves before using
WM_MDISETMENU to ensure that the last error can only be set after its return
if it was really done by the code handling it, i.e. if an error really
happened.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@78056 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775