message: Small fix in StaticStringBuilder
  Summary:
  If someone used less than the full capacity of the stringbuilder, it would have
  put garbage or crashed. This is more robust. Now, `str()` only reads strings
  that were actually put in.
diff: @@ -48,8 +48,8 @@ class StaticStringBuilder {
  std::string str() const {
  std::string result;
  result.reserve(m_total_chars);
  - for (const auto& s : m_strings) {
  - result += s;
  + for (int i = 0; i < m_index; ++i) {
  + result += m_strings[i];
  }
  return result;
  }
  