Function reserve

src/ArduinoJson/Memory/StringBuffer.hpp:20–33

  }

  char* reserve(size_t capacity) {
    if (node_ && capacity > node_->length) {
      // existing buffer is too small, we need to reallocate
      resources_->destroyString(node_);
      node_ = nullptr;
    }
    if (!node_)
      node_ = resources_->createString(capacity);
    if (!node_)
      return nullptr;
    size_ = capacity;
    node_->data[capacity] = 0;  // null-terminate the string
    return node_->data;
  }

  JsonString str() const {
    ARDUINOJSON_ASSERT(node_ != nullptr);