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):

i++;
      }
      if (this.toUnicode) {
        const charCode = getCharCode(unicode);
        if (charCode !== -1) {
          if (hasCurrentBufErrors()) {
            buffers.push(currentBuf.join(""));
            currentBuf.length = 0;
          }
          const charCodeLength = this.cMap
            ? this.cMap.getCharCodeLength(charCode)
            : 1;
          for (let j = charCodeLength - 1; j >= 0; j--) {
            currentBuf.push(String.fromCharCode((charCode >> (8 * j)) & 0xff));
          }
          continue;
        }
      }
      
      if (!hasCurrentBufErrors()) {
        buffers.push(currentBuf.join(""));
        currentBuf.length = 0;
      }
      currentBuf.push(String.fromCodePoint(unicode));
    }
    buffers.push(currentBuf.join(""));
    return buffers;
  }
}
class ErrorFont {