Attach (recommended) or Link to PDF file here:
[双引号不正常.pdf](https://github.com/mozilla/pdf.js/files/3533108/default.pdf)


Configuration:
- Web browser and its version:  IE11 , chrome 
- Operating system and its version:  win7
- PDF.js version:  master
- Is a browser extension: nothing 
teps to reproduce the problem:
1. viewer.html web  online preview


What is the expected behavior? (add screenshot)
Displayed in normal characters.

What went wrong? (add screenshot)
![20190823143813](https://user-images.githubusercontent.com/6269307/63571860-062e6600-c5b4-11e9-9fff-75d65cf6bade.png)

Not only the two characters I took screenshots, but also a lot of garbled Chinese punctuation. For example, the Chinese 《》 symbol. But these garbled characters only appear in some specific very old pdf files.

Link to a viewer (if hosted on a site other than mozilla.github.io/pdf.js or as Firefox/Chrome extension):nothing 

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);
      }