pull-job: always implicitly NUL terminate downloaded payload stored in memory

pull-job: always implicitly NUL terminate downloaded payload stored in memory

Just as a safety measure, let's always NUL terminate what we are
downloading, maybe future code will parse it as string, and is sloppy by
accident.

(We have similar logic in read_full_file(), and I think it's a really
good rule, to always implicitly NUL terminate blobs we acquire that
might very well be used as text later on)