Clear unused bytes in ia32_osendsig().
Mirror the fix for the native i386 implementation from r218327. This
code is compiled only when the non-default COMPAT_43 option is
configured.
Reported by: Ilja Van Sprundel <ivansprundel@ioactive.com>
Reviewed by: kib
MFC after: 3 days
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D18298