RFR: JDK-8077414 PSPromotionLAB _state is unintialized

[Eric Caspole]

While working on another bug I noticed that PSPromotionLAB field _state 
is uninitialized, and since I was staring at this code I wondered if 
this was part of the problem. I don't think so but I would like to 
initialize it so no one wonders in the future. I initialize it to 
"zero_size" since it is otherwise filled in with NULL ptrs.

Bug:
  https://bugs.openjdk.java.net/browse/JDK-8077414

Webrev:
  http://cr.openjdk.java.net/~ecaspole/JDK-8077414/00/webrev/

Passes JPRT.
Thanks,
Eric