All but one invocations of G1CollectedHeap::in_cset_fast_test already assume or make sure that the referenced object is within the heap (since the perm gen removal).
This additional check is superfluous, and decreases performance.
Change G1CollectedHeap::in_cset_fast_test to not assume that the passed references are NULL references.
The only place where NULL references were possible, is G1ParCopyClosure::do_oop_work(). However extracting the check out allows further code simplifications.