When auxiliary_device_add() returns error and then calls
auxiliary_device_uninit(), callback function adev_release calls kfree(madev).
We shouldn't call kfree(madev) again in the error handling path. Set 'madev'
to NULL.

Signed-off-by: Ma Ke <redacted>
---
Changes in v2:
- streamlined the patch according suggestions;
- revised the description.

The change is ok, but the commit message is missing a "Fixes" tag/line.
- Konstantin