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):

offsetIndex = segment.offsetIndex;
          for (j = start; j <= end; j++) {
            if (j === 0xffff) {
              continue;
            }
            glyphId = offsetIndex < 0 ? j : offsets[offsetIndex + j - start];
            glyphId = (glyphId + delta) & 0xffff;
            mappings.push({
              charCode: j,
              glyphId,
            });
          }
        }
      } else if (format === 6) {
        
        
        
        
        
        const firstCode = file.getUint16();
        const entryCount = file.getUint16();
        for (j = 0; j < entryCount; j++) {
          glyphId = file.getUint16();
          const charCode = firstCode + j;
          mappings.push({
            charCode,
            glyphId,
          });
        }
      } else {