makefs: correct El Torito bood record

The boot catalog pointer is a DWord, but we previously populated it via
cd9660_bothendian_dword which overwrote four unused bytes following it.
See El Torito 1.0 (1995) Figure 7 for details.

PR: 203531
Reported by: Coverity Scan
Reported by: Thomas Schmitt <scdbackup@gmx.net>
Reviewed by: kevans
CID: 977470
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D39231