6696264: assert("narrow oop can never be zero") for GCBasher & ParNewGC

Decouple set_klass() with zeroing the gap when compressed.

Reviewed-by: kvn, ysr, jrose

hotspot/src/share/vm/interpreter/bytecodeInterpreter.cpp

@@ -1931,6 +1931,7 @@ run:
               } else {
                 result->set_mark(markOopDesc::prototype());
               }
+              result->set_klass_gap(0);
               result->set_klass(k_entry);
               SET_STACK_OBJECT(result, 0);
               UPDATE_PC_AND_TOS_AND_CONTINUE(3, 1);