integrity: check the return value of audit_log_start()

audit_log_start() returns audit_buffer pointer on success or NULL on
error, so it is better to check the return value of it.

Fixes: 3323eec9 ("integrity: IMA as an integrity service provider")
Signed-off-by: NXiaoke Wang <xkernel.wang@foxmail.com>
Cc: <stable@vger.kernel.org>
Reviewed-by: NPaul Moore <paul@paul-moore.com>
Signed-off-by: NMimi Zohar <zohar@linux.ibm.com>

security/integrity/integrity_audit.c

@@ -45,6 +45,8 @@ void integrity_audit_message(int audit_msgno, struct inode *inode,
 		return;
 
 	ab = audit_log_start(audit_context(), GFP_KERNEL, audit_msgno);
+	if (!ab)
+		return;
 	audit_log_format(ab, "pid=%d uid=%u auid=%u ses=%u",
 			 task_pid_nr(current),
 			 from_kuid(&init_user_ns, current_uid()),