change where const length is set

Author: huangminghuang

src/mfast/coder/decoder_v2/fast_decoder_core.h

@@ -313,7 +313,7 @@ void fast_decoder_base::decode_field(const T &ext_ref, constant_operator_tag,
     // A field will not occupy any bit in the presence map if it is mandatory
     // and has the
     // constant operator.
-    // mref.to_initial_value();
+    mref.to_initial_value();
   }
   if (ext_ref.previous_value_shared())
     save_previous_value(mref);

src/mfast/ext_ref.h

@@ -310,12 +310,8 @@ class ext_mref<sequence_mref, LengthExtRef, ElementExtRef> {
   cref_type get() const { return base_; }
   is_optional_type optional() const { return is_optional_type(); }
   length_type set_length(value_storage &storage) const {
-    auto* length_inst = base_.instruction()->length_instruction();
     field_mref_base length_mref(nullptr, &storage,
-                                length_inst);
-    // a temporary storage is used for sequence length field
-    // and we must initialize it properly to support constant operators
-    length_inst->construct_value(storage, nullptr);
+                                base_.instruction()->length_instruction());
     return length_type(length_mref);
   }