fix leak when $LANG unset

fix leak when $LANG unset

The following leaked:

    LANG=  perl -e1

because in S_emulate_setlocale(), it was
1) making a copy of $ENV{"LANG"};
2) throwing that copy away and replacing it with "C" when it discovered
   that the string was empty.

A little judicious reordering of that chunk of code makes the issue go
away.

Showed up as failures of lib/locale_threads.t under valgrind / ASan.