SEGV in G1PLABAllocator

While trying to reproduce JDK-8151256, this SEGV occurred once in about 50K -Xcomp runs:
   -Xcomp -XX:+UnlockCommercialFeatures -XX:+ResourceManagement -verbose:gc HeapRetentionTest

When attempting plab allocation, we seem to have a null region pointer:

#8 <signal handler called>
#9 top (this=0x0) at /scratch/workspace/9-2-build-linux-amd64-phase2/jdk9/4520/hotspot/src/share/vm/gc/g1/heapRegion.hpp:138
#10 par_allocate_impl (actual_size=<optimized out>, desired_word_size=4374, min_word_size=<optimized out>, this=0x0)
    at /scratch/workspace/9-2-build-linux-amd64-phase2/jdk9/4520/hotspot/src/share/vm/gc/g1/heapRegion.inline.hpp:56
#11 par_allocate_no_bot_updates (actual_word_size=<optimized out>, desired_word_size=4374, min_word_size=<optimized out>, this=0x0)
    at /scratch/workspace/9-2-build-linux-amd64-phase2/jdk9/4520/hotspot/src/share/vm/gc/g1/heapRegion.inline.hpp:160
#12 par_allocate (bot_updates=false, actual_word_size=<optimized out>, desired_word_size=4374, min_word_size=<optimized out>, alloc_region=0x0)
    at /scratch/workspace/9-2-build-linux-amd64-phase2/jdk9/4520/hotspot/src/share/vm/gc/g1/g1AllocRegion.inline.hpp:65
#13 attempt_allocation (bot_updates=false, actual_word_size=<optimized out>, desired_word_size=4374, min_word_size=<optimized out>, this=<optimized out>)
    at /scratch/workspace/9-2-build-linux-amd64-phase2/jdk9/4520/hotspot/src/share/vm/gc/g1/g1AllocRegion.inline.hpp:85
#14 survivor_attempt_allocation (context=3 '\003', actual_word_size=<optimized out>, desired_word_size=4374, min_word_size=<optimized out>,
    this=0x7f405c051d30) at /scratch/workspace/9-2-build-linux-amd64-phase2/jdk9/4520/hotspot/src/share/vm/gc/g1/g1Allocator.cpp:198
#15 G1Allocator::par_allocate_during_gc (this=0x7f405c051d30, dest=..., min_word_size=<optimized out>, desired_word_size=4374,
    actual_word_size=<optimized out>, context=3 '\003')
    at /scratch/workspace/9-2-build-linux-amd64-phase2/jdk9/4520/hotspot/src/share/vm/gc/g1/g1Allocator.cpp:179
---Type <return> to continue, or q <return> to quit---
#16 0x00007f406456b94d in G1PLABAllocator::allocate_direct_or_new_plab (this=0x7f3f10015090, dest=dest@entry=..., word_sz=word_sz@entry=6,
    context=context@entry=3 '\003', plab_refill_failed=plab_refill_failed@entry=0x7f406273cb5e)
    at /scratch/workspace/9-2-build-linux-amd64-phase2/jdk9/4520/hotspot/src/share/vm/gc/g1/g1Allocator.cpp:272