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

originalData,
  xrefInfo,
  newRefs,
  xref = null,
  datasetsRef = null,
}) {
  updateXFA(datasetsRef, newRefs, xref);
  const newXref = new Dict(null);
  const refForXrefTable = xrefInfo.newRef;
  let buffer, baseOffset;
  const lastByte = originalData[originalData.length - 1];
  if (lastByte ===  0x0a || lastByte ===  0x0d) {
    buffer = [];
    baseOffset = originalData.length;
  } else {
    
    buffer = ["\n"];
    baseOffset = originalData.length + 1;
  }
  newXref.set("Size", refForXrefTable.num + 1);
  newXref.set("Prev", xrefInfo.startXRef);
  newXref.set("Type", Name.get("XRef"));
  if (xrefInfo.rootRef !== null) {
    newXref.set("Root", xrefInfo.rootRef);
  }
  if (xrefInfo.infoRef !== null) {
    newXref.set("Info", xrefInfo.infoRef);