Ensure that wxCmdLineParser::FoundSwitch() asserts when used wrongly.

This makes CmdLine unit test pass with the assertions backported from trunk,
where FoundSwitch() does assert when called for a non-switch.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_3_0_BRANCH@76915 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775