Add guard to prevent nil error while parsing octal notation

[jkowens]

While parsing a PDF, we ran into an issue where the the following error was raised:

NoMethodError: undefined method between? for nil:NilClass

We tracked it down to this line in the _parse_ method of PDFParser:

rep = (rep << 3) + (str_bytes.shift-48) if str_bytes[0].between?(48, 57)

It turns out there is potentially an error scenario while looping until str_bytes is empty where accessing str_bytes[0] would be a nil value. We just added the safe navigation operator which was added in Ruby 2.3. If this project is still supporting older rubies we can amend the commit.

[boazsegev]

Thanks!

Nicely Done! 🎉👏🏻👏🏻👏🏻