OPENAFS-SA-2024-002: verify FetchACL returned only a string

OPENAFS-SA-2024-002: verify FetchACL returned only a string

CVE-2024-10396

Supplement the previous commit by additionally verifying that
the returned ACL string occupies the entire XDR opaque, rejecting
any values returned that have an internal NUL prior to the end
of the opaque.

Reviewed-on: https://gerrit.openafs.org/15915
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Tested-by: Benjamin Kaduk <kaduk@mit.edu>
(cherry picked from commit 7e13414e8ea995d438cde3e60988225f3ab4cbcd)

Change-Id: I107f89e3d8a5c3c5cd67f6296742bfca7cace0e1
Reviewed-on: https://gerrit.openafs.org/15936
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Tested-by: Benjamin Kaduk <kaduk@mit.edu>