fix locale leaks on utf8 strings

fix locale leaks on utf8 strings

For example the following leaked:

    require POSIX; import POSIX ':locale_h';

    setlocale(&POSIX::LC_ALL, 'aa_DJ.iso88591') or die;
    use locale;

    my $ok = 'A' lt chr 0x100;

Some code in Perl__mem_collxfrm() does a couple of

    for (j = 1; j < 256; j++) { ... }

loops where for each chr(j) character it recursively calls itself, and
records the index of the 'smallest' / 'largest' result. However, when
updating cur_min_x / cur_max_x, it wasn't freeing the previous value.

The symptoms were that valgrind / Address Sanitizer found fault with
lib/locale.t