Panics (overflows) on parse fuzz target

[0x64617061]

I did more fuzz runs on the parse fuzz target and detected the following panics/issues:

• pdf/pdf/src/enc.rs:304:24 capacity overflow
• pdf/pdf/src/object/function.rs:152:100 attempt to subtract with overflow
• UNKNOWN stack overflow

I used code of the commit f87d3f5e90bddc9df8e0e144b06f2727e7afcc7e.

I attached panic inducing inputs the fuzzer generated that caused the respective panics:

pdf_panic_inputs.zip

[s3bk]

Acknowledged, thanks for finding it.
Unfortunately my todo list is overflowing also at the moment, so it may take a little time until I get around to fixing it.