Don't cast off_t to 32-bit type for output, bug fix

Don't cast off_t to 32-bit type for output, bug fix

off_t is most likely a 64-bit integer, so casting it to a 32-bit type
for output could lose data.  There are more issues like this in the
tree, but this is an instance where this could actually happen in
practice, since base backups are routinely larger than 4 GB.  So this
is separated out as a bug fix.

Reviewed-by: Heikki Linnakangas <hlinnaka@iki.fi>
Discussion: https://www.postgresql.org/message-id/flat/20ce62fa-47fc-457b-b504-12f3c1651726%40eisentraut.org