pgsql: Fix corner case with PGP decompression in pgcrypto

Fix corner case with PGP decompression in pgcrypto

A compressed stream may end with an empty packet, and PGP decompression
finished before reading this empty packet in the remaining stream.  This
caused a failure in pgcrypto, handling this case as corrupted data.
This commit makes sure to consume such extra data, avoiding a failure
when decompression the entire stream.  This corner case was reproducible
with a data length of 16kB, and existed since its introduction in
e94dd6a.  A cheap regression test is added to cover this case.

Thanks to Jeff Janes for the extra investigation.

Reported-by: Frank Gagnepain
Author: Kyotaro Horiguchi, Michael Paquier
Discussion: https://postgr.es/m/16476-692ef7b84e5fb893@postgresql.org
Backpatch-through: 9.5

Branch
------
REL_11_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/eceb415c6ff0eb2494f90397433a966a182f3908

Modified Files
--------------
contrib/pgcrypto/expected/pgp-compression.out | 30 +++++++++++++++++++++++++++
contrib/pgcrypto/pgp-compress.c               | 22 ++++++++++----------
contrib/pgcrypto/sql/pgp-compression.sql      | 21 +++++++++++++++++++
3 files changed, 62 insertions(+), 11 deletions(-)