pgsql: Fix bug in archive streamer with LZ4 decompression

Fix bug in archive streamer with LZ4 decompression

When decompressing some input data, the calculation for the initial
starting point and the initial size were incorrect, potentially leading
to failures when decompressing contents with LZ4.  These initialization
points are fixed in this commit, bringing the logic closer to what
exists for gzip and zstd.

The contents of the compressed data is clear (for example backups taken
with LZ4 can still be decompressed with a "lz4" command), only the
decompression part reading the input data was impacted by this issue.

This code path impacts pg_basebackup and pg_verifybackup, which can use
the LZ4 decompression routines with an archive streamer, or any tools
that try to use the archive streamers in src/fe_utils/.

The issue is easier to reproduce with files that have a low-compression
rate, like ones filled with random data, for a size of at least 512kB,
but this could happen with anything as long as it is stored in a data
folder.  Some tests are added based on this idea, with a file filled
with random bytes grabbed from the backend, written at the root of the
data folder.  This is proving good enough to reproduce the original
problem.

Author: Mikhail Gribkov <youzhick@gmail.com>
Discussion: https://postgr.es/m/CAMEv5_uQS1Hg6KCaEP2JkrTBbZ-nXQhxomWrhYQvbdzR-zy-wA@mail.gmail.com
Backpatch-through: 15

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/3369a3b49b0bc0a4205062e45623af297240c8c6

Modified Files
--------------
src/bin/pg_verifybackup/t/008_untar.pl        | 22 ++++++++++++++++++++++
src/bin/pg_verifybackup/t/010_client_untar.pl | 22 ++++++++++++++++++++++
src/fe_utils/astreamer_lz4.c                  |  4 ++--
3 files changed, 46 insertions(+), 2 deletions(-)