kio_sftp: Use proper boundary when queueing requests.

This fixes an off-by-one error when deciding whether the sftp_get code
path needs to make another request or not. This appears to be involved
in a data corruption bug (bug 312320) where file sizes that are a
multiple of the request buffer size (60*1024) end up getting an extra
full buffer of gibberish appended. E.g. a 61440-size file gets
downloaded to a 122880-sized file.

The bug report and proposed fix were both provided by Peter Klotz. I do
not yet understand at this point why this off-by-one error would cause
the extra packet to be appended, but I can confirm that the fix works.
It's at least easy enough to prove that a valid offset will not be == to
the file size (compare with the test on line 1151, for instance), and I
can confirm the fix seems to work.

It still doesn't make sense to me that the bug would be here though;
reading from an sftp channel that's already at EOF just returns 0 bytes,
and our code appears to handle that condition properly. In fact, the
check that is being corrected happens only *after* unconditionally
queueing up a first request packet, so you'd think the second EOF
response wouldn't even be an issue. Yet here we are.


FIXED-IN:4.10