Fix a missing write barrier to mandatory_only_iseq

Found by wbcheck

Author: jhawthorn

compile.c

@@ -9151,12 +9151,13 @@ compile_builtin_mandatory_only_method(rb_iseq_t *iseq, const NODE *node, const N
 
     VALUE ast_value = rb_ruby_ast_new(RNODE(&scope_node));
 
-    ISEQ_BODY(iseq)->mandatory_only_iseq =
+    const rb_iseq_t *mandatory_only_iseq =
       rb_iseq_new_with_opt(ast_value, rb_iseq_base_label(iseq),
                            rb_iseq_path(iseq), rb_iseq_realpath(iseq),
                            nd_line(line_node), NULL, 0,
                            ISEQ_TYPE_METHOD, ISEQ_COMPILE_DATA(iseq)->option,
                            ISEQ_BODY(iseq)->variable.script_lines);
+    RB_OBJ_WRITE(iseq, &ISEQ_BODY(iseq)->mandatory_only_iseq, (VALUE)mandatory_only_iseq);
 
     ALLOCV_END(idtmp);
     return COMPILE_OK;

prism_compile.c

@@ -3503,7 +3503,7 @@ pm_compile_builtin_mandatory_only_method(rb_iseq_t *iseq, pm_scope_node_t *scope
     pm_scope_node_init(&def.base, &next_scope_node, scope_node);
 
     int error_state;
-    ISEQ_BODY(iseq)->mandatory_only_iseq = pm_iseq_new_with_opt(
+    const rb_iseq_t *mandatory_only_iseq = pm_iseq_new_with_opt(
         &next_scope_node,
         rb_iseq_base_label(iseq),
         rb_iseq_path(iseq),
@@ -3515,6 +3515,7 @@ pm_compile_builtin_mandatory_only_method(rb_iseq_t *iseq, pm_scope_node_t *scope
         ISEQ_COMPILE_DATA(iseq)->option,
         &error_state
     );
+    RB_OBJ_WRITE(iseq, &ISEQ_BODY(iseq)->mandatory_only_iseq, (VALUE)mandatory_only_iseq);
 
     if (error_state) {
         RUBY_ASSERT(ISEQ_BODY(iseq)->mandatory_only_iseq == NULL);