Fix potential stack overflow in incremental backup.

Fix potential stack overflow in incremental backup.

The user can set RELSEG_SIZE to a high number at compile time, so we
can't use it to control the size of an array on the stack: it could be
many gigabytes in size.  On closer inspection, we don't really need that
intermediate array anyway.  Let's just write directly into the output
array, and then perform the absolute->relative adjustment in place.
This fixes new code from commit dc212340058.

Reviewed-by: Robert Haas <robertmhaas@gmail.com>
Discussion: https://postgr.es/m/CA%2BhUKG%2B2hZ0sBztPW4mkLfng0qfkNtAHFUfxOMLizJ0BPmi5%2Bg%40mail.gmail.com