8350630: [lworld] ObjBufferAllocator::initialize fails with assert(has_klass_gap()) failed: precondition

Author: fparain

src/hotspot/share/gc/shared/memAllocator.cpp

@@ -390,7 +390,7 @@ oop ObjAllocator::initialize(HeapWord* mem) const {
 }
 
 oop ObjBufferAllocator::initialize(HeapWord* mem) const {
-  oopDesc::set_klass_gap(mem, 0);
+  mem_clear(mem);
   return finish(mem);
 }