6774607: SIGSEGV or (!is_null(v),"oop value can never be zero") assertion when...

6774607: SIGSEGV or (!is_null(v),"oop value can never be zero") assertion when running with CMS and COOPs
Summary: Use the more permissive set_klass_or_null() and klass_or_null() interfaces in ParNew's workqueue overflow code that manipulates the klass-word.
Reviewed-by: coleenp

src/share/vm/gc_implementation/parNew/parNewGeneration.cpp

@@ -1205,7 +1205,7 @@ ParNewGeneration::take_from_overflow_list(ParScanThreadState* par_scan_state) {
   int n = 0;
   while (cur != NULL) {
     oop obj_to_push = cur->forwardee();
-    oop next        = oop(cur->klass());
+    oop next        = oop(cur->klass_or_null());
     cur->set_klass(obj_to_push->klass());
     if (par_scan_state->should_be_partially_scanned(obj_to_push, cur)) {
       obj_to_push = cur;

src/share/vm/oops/oop.inline.hpp

@@ -92,7 +92,7 @@ inline void oopDesc::set_klass_to_list_ptr(oop k) {
   // This is only to be used during GC, for from-space objects, so no
   // barrier is needed.
   if (UseCompressedOops) {
-    _metadata._compressed_klass = encode_heap_oop_not_null(k);
+    _metadata._compressed_klass = encode_heap_oop(k);  // may be null (parnew overflow handling)
   } else {
     _metadata._klass = (klassOop)k;
   }