Use the length of the buffer instead of recomputing it again

Micro optimization: use the already known buffer length instead of
calling wcslen() to compute it again.

include/wx/string.h

@@ -3645,7 +3645,7 @@ struct wxStringAsBufHelper<wchar_t>
     {
         wxScopedWCharBuffer wbuf(s.wc_str());
         if ( len )
-            *len = wxWcslen(wbuf);
+            *len = wbuf.length();
         return wbuf;
     }
 };