8076325: java hangs with -XX:ParallelGCThreads=0 -XX:+ExplicitGCInvokesConcurrent options

Summary: Added a guard of gc workers > 0 to execute logic.
Reviewed-by: stefank, mgerdin

src/share/vm/gc_implementation/g1/g1RootProcessor.cpp

@@ -93,19 +93,21 @@ void G1RootProcessor::worker_has_discovered_all_strong_classes() {
   uint n_workers = _g1h->n_par_threads();
   assert(ClassUnloadingWithConcurrentMark, "Currently only needed when doing G1 Class Unloading");
 
+  if (n_workers > 0) {
     uint new_value = (uint)Atomic::add(1, &_n_workers_discovered_strong_classes);
     if (new_value == n_workers) {
       // This thread is last. Notify the others.
       MonitorLockerEx ml(&_lock, Mutex::_no_safepoint_check_flag);
       _lock.notify_all();
     }
+  }
 }
 
 void G1RootProcessor::wait_until_all_strong_classes_discovered() {
   uint n_workers = _g1h->n_par_threads();
   assert(ClassUnloadingWithConcurrentMark, "Currently only needed when doing G1 Class Unloading");
 
-  if ((uint)_n_workers_discovered_strong_classes != n_workers) {
+  if (n_workers > 0 && (uint)_n_workers_discovered_strong_classes != n_workers) {
     MonitorLockerEx ml(&_lock, Mutex::_no_safepoint_check_flag);
     while ((uint)_n_workers_discovered_strong_classes != n_workers) {
       _lock.wait(Mutex::_no_safepoint_check_flag, 0, false);