ascii85Decode fails on valid input

ascii85 decoding can fail [here](https://github.com/pdfcpu/pdfcpu/blob/v0.9.1/pkg/filter/ascii85Decode.go#L63-L70), if the sequence has both a carriage return _and_ a newline appended, since only the second character will be stripped.