C - Copy List w/Sentinel NULL

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

/** realloc 'ptr' of 'nelem' of 'psz' to 'nelem * 2' of 'psz'.
 *  returns pointer to reallocated block of memory with new
 *  memory initialized to 0/NULL. return must be assigned to
 *  original pointer in caller.
 */
void *xrealloc2 (void *ptr, size_t psz, size_t *nelem)
{   void *memptr = realloc ((char *)ptr, *nelem * 2 * psz);
    if (!memptr) {
        perror ("realloc(): virtual memory exhausted.");
        exit (EXIT_FAILURE);
    }   /* zero new memory (optional) */
    memset ((char *)memptr + *nelem * psz, 0, *nelem * psz);
    *nelem *= 2;
    return memptr;
}

char **copy_sentinel_array (char **list)
{
    char **buffer = NULL;
    size_t avail = 2, used = 0;

    /* allocate/validate 2 pointers initially */
    if (!(buffer = calloc (avail, sizeof *buffer))) {
        perror ("calloc-buffer");
        exit (EXIT_FAILURE);
    }

    while (*list) {                                 /* for each word in list */
        size_t len = strlen (*list);                /* get length */
        if (!(buffer[used] = malloc (len + 1))) {   /* allocate storage */
            perror ("malloc-buffer[used]");
            exit (EXIT_FAILURE);
        }
        memcpy (buffer[used], *list, len + 1);      /* copy list->buffer */
        if (++used == avail)    /* is realloc req'd? (save room for NULL) */
            buffer = xrealloc2 (buffer, sizeof *buffer, &avail);
        list++;             /* increment list */
    }
    buffer[used] = NULL;    /* set sentinel NULL */

    return buffer;          /* return buffer */
}

int main (void) {

    char *list[] =  { "My", "Dog", "Has", "Fleas", "My", "Cat",
                        "Has", "None", "--", "Lucky", "Cat", NULL };
    char **buf = copy_sentinel_array (list);    /* copy list->buf */
    size_t n = 0;

    while (buf[n]) {
        printf (" %s", buf[n]);
        free (buf[n++]);
    }
    free (buf);
    putchar ('\n');

    return 0;
}