libadmin/vos/vosutils.c: mask out sign-extension

Right-shifting a signed int by 24 bits can produce a value outside of
0..0xff due to sign-extension.  As a result, in AddressMatch(), the
first bPattern!=255 check can never succeed.  Fix by masking with 255
before comparison.

Change-Id: Idb0b4c176ff120c7cf0e03a935ebfdca51084bbd
Reviewed-on: http://gerrit.openafs.org/8884
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>