Fix packet partial body length

I can confirm https://github.com/openpgpjs/openpgpjs/issues/123

Only have a private message to test with. The message contains a compressed packets with a signature packet that was not readable and failed at:
```
// some sanity checks
    if (input === null || input.length <= position || input.substring(position).length < 2 || (input.charCodeAt(position) &
      0x80) === 0) {
      throw new Error("Error during parsing. This message / key is probably not containing a valid OpenPGP format.");
    }
```