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: Xiaoke Wang <xkernel.wang@foxmail.com>
Cc: <stable@vger.kernel.org>
Reviewed-by: Paul Moore <paul@paul-moore.com>
Signed-off-by: Mimi Zohar <zohar@linux.ibm.com>