Attach (recommended) or Link to PDF file here:

Configuration:
- Web browser and its version: Chrome 63.0.3239.132 
- Operating system and its version: Windows 10
- PDF.js version: using your official demo project
- Is a browser extension: No

Steps to reproduce the problem:
1.  Just load attached pdf
2.

What is the expected behavior? (add screenshot)
[E-WG18_105147.pdf](https://github.com/mozilla/pdf.js/files/1634833/E-WG18_105147.pdf)

What went wrong? (add screenshot)
![image](https://user-images.githubusercontent.com/29802177/34986271-3ca71a5c-fab7-11e7-95e8-1739dda3cd04.png)

Link to a viewer (if hosted on a site other than mozilla.github.io/pdf.js or as Firefox/Chrome extension):

const cid = map[charCode];
          if (cidToGidMap[cid] !== undefined) {
            map[+charCode] = cidToGidMap[cid];
          }
        }
        
        
        if (
          cidToGidMap.length !== this.toUnicode.length &&
          properties.hasIncludedToUnicodeMap &&
          this.toUnicode instanceof IdentityToUnicodeMap
        ) {
          this.toUnicode.forEach(function (charCode, unicodeCharCode) {
            const cid = map[charCode];
            if (cidToGidMap[cid] === undefined) {
              map[+charCode] = unicodeCharCode;
            }
          });
        }
      }
      if (!(this.toUnicode instanceof IdentityToUnicodeMap)) {
        this.toUnicode.forEach(function (charCode, unicodeCharCode) {
          map[+charCode] = unicodeCharCode;
        });
      }
      this.toFontChar = map;
      this.toUnicode = new ToUnicodeMap(map);
    } else if (/Symbol/i.test(fontName)) {
      this.toFontChar = buildToFontChar(
        SymbolSetEncoding,