iio: inkern: fix a NULL dereference on error

In twl4030_bci_probe() there are some failure paths where we call
iio_channel_release() with a NULL pointer.  (Apparently, that driver can
opperate without a valid channel pointer).  Let's fix it by adding a
NULL check in iio_channel_release().

Fixes: 2202e1fc ('drivers: power: twl4030_charger: fix link problems when building as module')
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>

drivers/iio/inkern.c

@@ -351,6 +351,8 @@ EXPORT_SYMBOL_GPL(iio_channel_get);
 
 void iio_channel_release(struct iio_channel *channel)
 {
+	if (!channel)
+		return;
 	iio_device_put(channel->indio_dev);
 	kfree(channel);
 }