drm: fix blob pointer check

[Dan Carpenter]

On Fri, Mar 11, 2016 at 11:52:44AM +0000, Lionel Landwerlin wrote:
> Hi Dan,
> 
> Thanks a lot for pointing this out. I saw you previous comment but
> didn't realize the issue.
> I'll set the pointer to NULL.
> 
> I don't if there is an agreement on this, but do you think the
> unref/free functions should check for error pointers?

I'm with the netdev people on this and think we shouldn't add sanity
checks to free functions.  For example, free_netdev().  If you always
keep track of what is allocated and what is not then the code is easier
to follow than if you start mixing them up and passing invalid pointers
to other functions.  If you handle errors right away that's simpler.

regards,
dan carpenter