Attach (recommended) or Link to PDF file here:
[chinese.pdf](https://github.com/mozilla/pdf.js/files/6404389/download.1.pdf)

Configuration:
- Web browser and its version:90.0.4430.93
- Operating system and its version: mac 10.15.5
- PDF.js version: 2.9.146
- Is a browser extension:no

Steps to reproduce the problem:
1. The Chinese in the attachment cannot be rendered

Brower ✅
![image](https://user-images.githubusercontent.com/13477114/116678074-81782a00-a9db-11eb-8719-74f48fcebc2b.png)

PDF.js ❌ [https://mozilla.github.io/pdf.js/web/viewer.html](https://mozilla.github.io/pdf.js/web/viewer.html)
![image](https://user-images.githubusercontent.com/13477114/116678212-a371ac80-a9db-11eb-847f-dd1ad7752c13.png)


What is the expected behavior? (add screenshot)
**The Chinese character can be rendered**
What went wrong? (add screenshot)
**All of the Chinese character cannot be rendered**
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 {