RFR (XS): 8183127: UpdateRSetDeferred should not enqueue cards for NULL references

[Thomas Schatzl]

Hi,

  can I have reviews for this change that adds a NULL-check in the
UpdateRSetDeferred closure so that we do not enqueue cards with NULL
references in it during evacuation failure?

CR:
https://bugs.openjdk.java.net/browse/JDK-8183127
Webrev:
http://cr.openjdk.java.net/~tschatzl/8183127/webrev/
Testing:
jprt, gcbasher with G1EvacuationFailureALot for 1/2 hour

I think this amount of testing is sufficient as the reasoning for this
change is not *that* complicated.

Thanks,
  Thomas