RFR: Add missing null check in assert

[Aleksey Shipilev]

On 03/28/2017 10:46 PM, Roman Kennke wrote:
> the GCBasher test started to fail with my last update-references change.
> It's caused by an attempt to resolve the brooks ptr of an object that
> may become NULL in an assert. I added a null-check there to avoid the
> problem.
> 
> http://cr.openjdk.java.net/~rkennke/missingnullcheck/webrev.00/

OK.

-Aleksey