Fix/invalid xref

[brandnday]

What?

This PR is to address issue in https://github.com/cantoo-scribe/pdf-lib/issues/64 It is found that there is no problem with the pdf document, but the xref section is corrupted.

Why?

Our team use this library and found this issue when we upload pdf with invalid xref. Turns out this issue raised before

How?

It will precheck if the digits in xref object is valid, if it is not valid, it will just return the empty xref

Testing?

Ran the test locally

New Dependencies?

Screenshots

Suggested Reading?

Anything Else?

Checklist

• [x] I read CONTRIBUTING.md.
• [x] I read MAINTAINERSHIP.md#pull-requests.
• [ ] I added/updated unit tests for my changes.
• [ ] I added/updated integration tests for my changes.
• [ ] I ran the integration tests.
• [ ] I tested my changes in Node, Deno, and the browser.
• [ ] I viewed documents produced with my changes in Adobe Acrobat, Foxit Reader, Firefox, and Chrome.
• [ ] I added/updated doc comments for any new/modified public APIs.
• [ ] My changes work for both new and existing PDF files.
• [ ] I ran the linter on my changes.