review request (XS): 6978300 G1 debug builds crash if ParGCThreads==0

[John Coomes]

I'd appreciate reviews of this small change:

http://cr.openjdk.java.net/~jcoomes/6978300-taskqueue-g1-gcthreads/

Debug builds of G1 crash when ParallelGCThreads==0 because of a null
pointer deref.  It was introduced by my changes for 6966222.

-John