Simplify rb_obj_set_fields

No need to handle `fields_obj == 0`.

Author: byroot

variable.c

@@ -1255,21 +1255,16 @@ rb_obj_set_fields(VALUE obj, VALUE fields_obj, ID field_name, VALUE original_fie
 {
     ivar_ractor_check(obj, field_name);
 
-    RUBY_ASSERT(!fields_obj || IMEMO_TYPE_P(fields_obj, imemo_fields));
+    RUBY_ASSERT(IMEMO_TYPE_P(fields_obj, imemo_fields));
     RUBY_ASSERT(!original_fields_obj || IMEMO_TYPE_P(original_fields_obj, imemo_fields));
 
     if (fields_obj != original_fields_obj) {
-        if (fields_obj) {
-            RB_VM_LOCKING() {
-                st_insert(generic_fields_tbl_, (st_data_t)obj, (st_data_t)fields_obj);
-            }
-
-            RB_OBJ_WRITTEN(obj, original_fields_obj, fields_obj);
-        }
-        else {
-            rb_free_generic_ivar(obj);
+        RB_VM_LOCKING() {
+            st_insert(generic_fields_tbl_, (st_data_t)obj, (st_data_t)fields_obj);
         }
 
+        RB_OBJ_WRITTEN(obj, original_fields_obj, fields_obj);
+
         if (original_fields_obj) {
             // Clear root shape to avoid triggering cleanup such as free_object_id.
             rb_imemo_fields_clear(original_fields_obj);