Fix `ZeroCopyCodedInputStream::ReadCord()`

`ZeroCopyCodedInputStream::ReadCord()` is implemented in terms of `CodedInputStream::ReadCord()` which is incorrect: the contract for the interface method `ZeroCopyInputStream::ReadCord()` is that it appends to the provided cord output where `CodedInputStream::ReadCord()` sets the cord output value, clearing any pre-existing contents. This change fixes the internal implementation to append to any pre-existing contents.

PiperOrigin-RevId: 500872467

src/google/protobuf/message_lite.cc

@@ -204,8 +204,15 @@ class ZeroCopyCodedInputStream : public io::ZeroCopyInputStream {
   bool aliasing_enabled() { return cis_->aliasing_enabled_; }
 
   bool ReadCord(absl::Cord* cord, int count) final {
+    // Fast path: tail call into ReadCord reading new value.
+    if (PROTOBUF_PREDICT_TRUE(cord->empty())) {
       return cis_->ReadCord(cord, count);
     }
+    absl::Cord tmp;
+    bool res = cis_->ReadCord(&tmp, count);
+    cord->Append(std::move(tmp));
+    return res;
+  }
  private:
   io::CodedInputStream* cis_;
 };