[Fix] Add defensive checks to PDF parser for malformed input

[Fix] Add defensive checks to PDF parser for malformed input

Add pcall wrappers and type checks throughout pdf.lua to handle
malformed PDFs from untrusted sources without crashing:

- Add nil checks for stream objects before accessing fields
- Wrap grammar matches in pcall to catch parsing errors
- Add type validation before ipairs calls on trie match results
- Wrap span extractions in pcall to handle invalid offsets
- Add defensive checks in processor functions (trailer, suspicious)
- Wrap URL creation in pcall for malformed URI strings

Errors are logged via debugm for diagnosis while allowing
processing to continue gracefully.