[media] cx25840: fix unchecked return values

In functions cx25840_initialize(), cx231xx_initialize(), and
cx23885_initialize(), the return value of create_singlethread_workqueue()
is used without validation. This may result in NULL dereference and cause
kernel crash. This patch fixes it.
Signed-off-by: Pan Bian <bianpan2016@163.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>

drivers/media/i2c/cx25840/cx25840-core.c

@@ -416,11 +416,13 @@ static void cx25840_initialize(struct i2c_client *client)
 	INIT_WORK(&state->fw_work, cx25840_work_handler);
 	init_waitqueue_head(&state->fw_wait);
 	q = create_singlethread_workqueue("cx25840_fw");
+	if (q) {
 		prepare_to_wait(&state->fw_wait, &wait, TASK_UNINTERRUPTIBLE);
 		queue_work(q, &state->fw_work);
 		schedule();
 		finish_wait(&state->fw_wait, &wait);
 		destroy_workqueue(q);
+	}
 
 	/* 6. */
 	cx25840_write(client, 0x115, 0x8c);
@@ -630,11 +632,13 @@ static void cx23885_initialize(struct i2c_client *client)
 	INIT_WORK(&state->fw_work, cx25840_work_handler);
 	init_waitqueue_head(&state->fw_wait);
 	q = create_singlethread_workqueue("cx25840_fw");
+	if (q) {
 		prepare_to_wait(&state->fw_wait, &wait, TASK_UNINTERRUPTIBLE);
 		queue_work(q, &state->fw_work);
 		schedule();
 		finish_wait(&state->fw_wait, &wait);
 		destroy_workqueue(q);
+	}
 
 	/* Call the cx23888 specific std setup func, we no longer rely on
 	 * the generic cx24840 func.
@@ -748,11 +752,13 @@ static void cx231xx_initialize(struct i2c_client *client)
 	INIT_WORK(&state->fw_work, cx25840_work_handler);
 	init_waitqueue_head(&state->fw_wait);
 	q = create_singlethread_workqueue("cx25840_fw");
+	if (q) {
 		prepare_to_wait(&state->fw_wait, &wait, TASK_UNINTERRUPTIBLE);
 		queue_work(q, &state->fw_work);
 		schedule();
 		finish_wait(&state->fw_wait, &wait);
 		destroy_workqueue(q);
+	}
 
 	cx25840_std_setup(client);