acl_copyin: avoid returning uninitialized memory

acl_copyin did not validate the return value of acl_copy_oldacl_into_acl
which could lead to uninitialized acl structure memory reads.

Reported by: Synacktiv
Reviewed by: markj, emaste
Sponsored by: The Alpha-Omega Project
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D46125

(cherry picked from commit 6ee6c7b146643170602091e8c330314e4ef47b42)