Don't create an invalid iterator in wxDateTime::ParseTime().

Don't create an invalid iterator in wxDateTime::ParseTime().

Creating an iterator pointing beyond the string end resulted in an assert from
MSVC 9 CRT. Fix this by using wxString ctor taking length (which may be
greater than the length of the string) instead of the one taking two iterators
(which must both be valid).

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@63017 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775