make_not_entrant_or_zombie sees zombies

make_not_entrant_or_zombie() is apparently being invoked on a zombie method. Presumably because of a race.

V [libjvm.so+0x15f5530] void VMError::report_and_die()+0x700;; __1cHVMErrorOreport_and_die6M_v_+0x700
V [libjvm.so+0xa71860] void report_vm_error(const char*,int,const char*,const char*)+0x70;; __1cPreport_vm_error6Fpkci11_v_+0x70
V [libjvm.so+0x123b7a4] bool nmethod::make_not_entrant_or_zombie(unsigned)+0x74;; __1cHnmethodbAmake_not_entrant_or_zombie6MI_b_+0x74
V [libjvm.so+0x14cbd48] int NMethodSweeper::process_nmethod(nmethod*)+0x468;; __1cONMethodSweeperPprocess_nmethod6FpnHnmethod__i_+0x468
V [libjvm.so+0x14caddc] void NMethodSweeper::sweep_code_cache()+0x894;; __1cONMethodSweeperQsweep_code_cache6F_v_+0x894
V [libjvm.so+0x14ca41c] void NMethodSweeper::possibly_sweep()+0x39c;; __1cONMethodSweeperOpossibly_sweep6F_v_+0x39c
V [libjvm.so+0x9e34d8] CompileTask*CompileQueue::get()+0x20;; __1cMCompileQdDueueDget6M_pnLCompileTask__+0x20
V [libjvm.so+0x9e6ea4] void CompileBroker::compiler_thread_loop()+0x22c;; __1cNCompileBrokerUcompiler_thread_loop6F_v_+0x22c
V [libjvm.so+0x153689c] void JavaThread::thread_main_inner()+0x26c;; __1cKJavaThreadRthread_main_inner6M_v_+0x26c
V [libjvm.so+0x15365b8] void JavaThread::run()+0x500;; __1cKJavaThreadDrun6M_v_+0x500
V [libjvm.so+0x12ab86c] java_start+0x3fc;; java_start+0x3fc
C [libc.so.1+0xe25bc] _lwp_start+0x8;; _lwp_start+0x8