Fix caching wrong length in wxString(str, len) ctor in UTF-8 build.

Fix caching wrong length in wxString(str, len) ctor in UTF-8 build.

A length greater than that of the source string could be passed to this ctor.
This worked correctly, i.e. created a string which was a copy of the source
one but cached a wrong length for it.

Avoid this by explicitly checking the length before caching it in
wxString::assign(str, len).

See #14130.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@70986 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775