pypdf has possible long runtimes for malformed startxref
### Impact
An attacker who exploits this vulnerability can craft a PDF which leads to possibly long runtimes for invalid `startxref` entries. When rebuilding the cross-reference table, PDF files with lots of whitespace characters become problematic. Only the non-strict reading mode is affected.
### Patches
This has been fixed in pypdf==6.6.0
https://github.com/py-pdf/pypdf/releases/tag/6.6.0.
### Workarounds
```python
from pypdf import PdfReader, PdfWriter
# Instead of
reader = PdfReader("file.pdf")
# use the strict mode:
reader = PdfReader("file.pdf", strict=True)
# Instead of
writer = PdfWriter(clone_from="file.pdf")
# use an explicit strict reader:
writer = PdfWriter(clone_from=PdfReader("file.pdf", strict=True))
```
### Resources
This issue has been fixed in #3594.