Concatenate multiple strings to a freshly allocated result

concat.c

/* concat(0) creates a new "" in heap memory,
   and concat(1, foo) is equivalent to strdup(foo).
	 
	 Note I haven't tested this function yet. */

char *concat(size_t n, ...)
{
	va_list ap;
	size_t retlen = 0;
	char *ret = calloc(1,1);
	if (!ret)
		return NULL;

	va_start(ap, n);
	while (n-- > 0)
	{
		char *s = va_arg(ap, char*);
		size_t slen = strlen(s);
		char *bigger = realloc(ret, retlen+slen+1);
		if (!bigger)
		{
			free(ret);
			return NULL;
		}
		else
			ret = bigger;
		strcpy(ret+retlen, s);
		retlen += slen;
	}
	va_end(ap);
	return ret;
}