fix out of bounds read in zlsb()

prev scales i from num chars -> num bits, then indexes with it, causing
a page fault or reading garbage.  scale i after the read instead.

here is a reproducer

    #include <stdio.h>
    #include "libzahl/zahl.h"

    int
    main(void)
    {
        z_t x;
        zinit(x);
        zsetu(x, 1);

        zlsh(x, x, 2097153);

        printf("used chars:  expect 32769, have %lu\n", x->used);

        size_t tz = zlsb(x);

        printf("tz:          expect 2097153, have %lu\n", tz);
    }

0 files changed, 0 insertions, 0 deletions