Fall back on ::GetMessagePos() if ::GetCursorPos() fails under MSW.

Fall back on ::GetMessagePos() if ::GetCursorPos() fails under MSW.

In some rare but reproducible cases GetCursorPos() can fail and return without
filling in the provided point. Fall back to GetMessagePos() if this happens:
this is not ideal but clearly better than using uninitialized position or hard
coding something like (0, 0).

Closes #13664.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@69758 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775