Attach (recommended) or Link to PDF file here: [ACORD 0211 2016-09 Acroform BLANK.pdf](https://github.com/mozilla/pdf.js/files/6353664/ACORD.0211.2016-09.Acroform.BLANK.pdf)

Configuration:
- Web browser and its version: Google Chrome Version 90.0.4430.72 (Official Build) (x86_64)
- Operating system and its version: Windows 10 Pro 20H2
- PDF.js version: 2.7.570
- Is a browser extension: No

Steps to reproduce the problem:
1. Load attached form into PDFViewer
2. Call saveDocument method on loaded document proxy

What is the expected behavior: should not crash

What went wrong? (add screenshot)
It crashes in writer.js. Line 273 as of 21 Apr 2021.
![image](https://user-images.githubusercontent.com/10527821/115615989-4d747900-a2f8-11eb-8e6a-fa3070d813a7.png)

Line that crashes: 
![image](https://user-images.githubusercontent.com/10527821/115616056-6846ed80-a2f8-11eb-8df1-b4633d3e5e8e.png)

Potential fix: 
![image](https://user-images.githubusercontent.com/10527821/115616270-af34e300-a2f8-11eb-82c9-c13bfd111fe4.png)

Potential fix is based on this line if worker.js file: 
![image](https://user-images.githubusercontent.com/10527821/115616437-e0151800-a2f8-11eb-947c-d89e72774bd5.png)


Link to a viewer (if hosted on a site other than mozilla.github.io/pdf.js or as Firefox/Chrome extension): not hosted

newXref.set("W", sizes);
  newXref.set("Length", tableLength);
  buffer.push(`${refForXrefTable.num} ${refForXrefTable.gen} obj\n`);
  writeDict(newXref, buffer, null);
  buffer.push(" stream\n");
  const bufferLen = buffer.reduce((a, str) => a + str.length, 0);
  const footer = `\nendstream\nendobj\nstartxref\n${baseOffset}\n%%EOF\n`;
  const array = new Uint8Array(
    originalData.length + bufferLen + tableLength + footer.length
  );
  
  array.set(originalData);
  let offset = originalData.length;
  
  for (const str of buffer) {
    writeString(str, offset, array);
    offset += str.length;
  }
  
  for (const [type, objOffset, gen] of xrefTableData) {
    offset = writeInt(type, sizes[0], offset, array);
    offset = writeInt(objOffset, sizes[1], offset, array);
    offset = writeInt(gen, sizes[2], offset, array);
  }
  
  writeString(footer, offset, array);
  return array;
}
export { incrementalUpdate, writeDict };