Method safeMid

Fleece/Support/diff_match_patch.hh:2361–2363

   */
 private:
  static inline string_t safeMid(const string_t &str, size_t pos) {
    return (pos == str.length()) ? string_t() : str.substr(pos);
  }

  /**
   * A safer version of string_t.mid(pos, len).  This one returns "" instead of