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):

let glyphs = this._charsCache[chars];
    if (glyphs) {
      return glyphs;
    }
    glyphs = [];
    if (this.cMap) {
      
      
      const c = Object.create(null),
        ii = chars.length;
      let i = 0;
      while (i < ii) {
        this.cMap.readCharCode(chars, i, c);
        const { charcode, length } = c;
        i += length;
        
        const glyph = this._charToGlyph(
          charcode,
          length === 1 && chars.charCodeAt(i - 1) === 0x20
        );
        glyphs.push(glyph);
      }
    } else {
      for (let i = 0, ii = chars.length; i < ii; ++i) {
        const charcode = chars.charCodeAt(i);
        const glyph = this._charToGlyph(charcode, charcode === 0x20);
        glyphs.push(glyph);
      }