When GC'ing a thread, assert that it has no active audit record.

This should not happen, but with this assert, brueffer and I would
not have spent 45 minutes trying to figure out why he wasn't
seeing audit records with the audit version in CVS.

Obtained from:	TrustedBSD Project

sys/kern/kern_thread.c

@@ -172,7 +172,9 @@ thread_dtor(void *mem, int size, void *arg)
 		/* NOTREACHED */
 	}
 #endif
-
+#ifdef AUDIT
+	audit_thread_free(td);
+#endif
 	free_unr(tid_unrhdr, td->td_tid);
 	sched_newthread(td);
 }

sys/security/audit/audit.c

@@ -1025,6 +1025,16 @@ audit_thread_alloc(struct thread *td)
 	td->td_ar = NULL;
 }
 
+/*
+ * Thread destruction.
+ */
+void
+audit_thread_free(struct thread *td)
+{
+
+	KASSERT(td->td_ar == NULL, ("audit_thread_free: td_ar != NULL"));
+}
+
 /* 
  * Initialize the audit information for the a process, presumably the first 
  * process in the system.

sys/security/audit/audit.h

@@ -180,6 +180,7 @@ void			 audit_proc_fork(struct proc *parent,
 					 struct proc *child);
 void			 audit_proc_free(struct proc *p);
 void			 audit_thread_alloc(struct thread *td);
+void			 audit_thread_free(struct thread *td);
 
 /*
  * Define a macro to wrap the audit_arg_* calls by checking the global