libutil: Make free_iovec() zero its arguments, like the manpage claims

Approved by: kib
Differential Revision: https://reviews.freebsd.org/D59929

lib/libutil/mntopts.c

@@ -319,4 +319,6 @@ free_iovec(struct iovec **iov, int *iovlen)
 	for (i = 0; i < *iovlen; i += 2)
 		free((*iov)[i].iov_base);
 	free(*iov);
+	*iov = NULL;
+	*iovlen = 0;
 }