By passing an empty file with a declared negative size,
a hostile VM can decrease the total bytes counter, while
not have do supply a huge amount of data, thus disabing
the byte size check, and potentially filling the target
filesystem.
Also do not rely on unpack being called just once if we don't
have to and initialize counts.
Since we don't know directory size before populating with files,
we just accumulate the size on the second pass, but do not actually
check for the limit being reached. If there's any file after that,
that'll trip the check.
