regression b08: SEGV in FastScanClosure::do_oop

nsk/jvmti/RetransformClasses/retransform001

Crashes with a SEGV:

Stack: [0xfc100000,0xfc180000), sp=0xfc17f4a0, free space=509k
Native frames: (J=compiled Java code, j=interpreted, Vv=VM code, C=native code)
V [libjvm.so+0x15c97c];; void FastScanClosure::do_oop(oopDesc**)+0x38
V [libjvm.so+0x48d1b8];; void SystemDictionary::shared_oops_do(OopClosure*)+0x5c
V [libjvm.so+0x4a8d78];; void Universe::oops_do(OopClosure*,bool)+0x2c
V [libjvm.so+0x457f40];; void SharedHeap::process_strong_roots(bool,SharedHeap::ScanningOption,OopClosure*,OopsInGenClosure*)+0x6c
V [libjvm.so+0x287c28];; void GenCollectedHeap::gen_process_strong_roots(int,bool,bool,SharedHeap::ScanningOption,OopsInGenClosure*,OopsInGenClosure*)+0x38
V [libjvm.so+0x264764];; void DefNewGeneration::collect(bool,bool,unsigned,bool)+0x3fc
V [libjvm.so+0x287510];; void GenCollectedHeap::do_collection(bool,bool,unsigned,bool,int)+0x73c
V [libjvm.so+0x230538];; HeapWord*GenCollectorPolicy::satisfy_failed_allocation(unsigned,bool)+0x194
V [libjvm.so+0x15bea4];; void VM_GenCollectForAllocation::doit()+0x9c
V [libjvm.so+0x13dad8];; void VM_Operation::evaluate()+0x88
V [libjvm.so+0x4e613c];; void VMThread::evaluate_operation(VM_Operation*)+0xd4
V [libjvm.so+0x4e66cc];; void VMThread::loop()+0x470
V [libjvm.so+0xb526c];; void VMThread::run()+0xa0
V [libjvm.so+0x413f08];; java_start+0x194

VM_Operation (0xfe77e4ec): generation collection for allocation, mode: safepoint, requested by thread 0x00031800

This seems to be a regression in b08. It occurs on all platforms and in both client and server VM - though client seems most common. Nightly failures in main/baseline trace back to around Feb 9. This failure doesn't seem to appear in other baselines.