Add missing WB for iseq

The write barrier wasn't being called for this object, so add the
missing WB.  Automatic compaction moved the reference because it didn't
know about the relationship (that's how I found the missing WB).

compile.c

@@ -8334,6 +8334,7 @@ iseq_compile_each0(rb_iseq_t *iseq, LINK_ANCHOR *const ret, const NODE *node, in
 	debugp_param("lit", node->nd_lit);
 	if (!popped) {
 	    ADD_INSN1(ret, line, putobject, node->nd_lit);
+            RB_OBJ_WRITTEN(iseq, Qundef, node->nd_lit);
 	}
 	break;
       }