Fix out of bounds string access in wxMSW wxDirDialog.

Fix out of bounds string access in wxMSW wxDirDialog.

Using the initial directory of "/" (or "\\" or in fact any string consisting
solely of slashes and backslashes) resulted in a crash as the code incorrectly
tried to read the character before the beginning of the string.

Fix this by checking that the string is not empty before using s.end()-1
iterator.

Closes #12946.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@66961 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775