Fix dereferencing invalid iterator in wxString in UTF-8 build

Fix iterator going past end of string in PosLenToImpl, it can't become
end() as the end iterator can't be dereferenced.

Closes #23305.

src/common/string.cpp

@@ -229,7 +229,7 @@ void wxString::PosLenToImpl(size_t pos, size_t len,
             // going beyond the end of the string, just as std::string does
             const const_iterator e(end());
             const_iterator i(b);
-            while ( len && i <= e )
+            while ( len && i < e )
             {
                 ++i;
                 --len;