OpenBSM: fix free() in au_read_rec error case

buf is a char ** and *buf is the allocated buffer.

PR: 267050
Reported by: Robert Morris <rtm@lcs.mit.edu>
MFC after: 3 days
Sponsored by: The FreeBSD Foundation

(cherry picked from commit c61b3f7de9a0a39d2bc78e3e78d148c369e706a5)