8030027: nsk/jvmti/scenarios/hotswap/HS101/hs101t006 Crashed the vm on Linux-amd64: SIGSEGV in JavaThread::last_java_vframe(RegisterMap*)+0xfa Summary: Add a safe guard to VMOp_GetCurrentLocation against non-walkable stack of target thread exiting from Java execution Reviewed-by: mgronlun, dholmes, sla, dcubed Contributed-by: serguei.spitsyn@oracle.com