Reset the system last error value before checking it in wxLog test.

Explicitly set the last error indicator to 0 before checking that
wxLogSysError() logs it correctly as it wasn't always set to 0 otherwise.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@76123 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775