pgsql: Fix handling of empty uncompressed posting list pages in GIN

Fix handling of empty uncompressed posting list pages in GIN

PostgreSQL 9.4 introduces posting list compression in GIN. This feature
supports online upgrade, so that after pg_upgrade uncompressed posting
lists are compressed on-the-fly. Underlying code appears to always
expect at least one item on uncompressed posting list page. But there
could be completely empty pages, because VACUUM never deletes leftmost
and rightmost pages from posting trees. This commit fixes that.

Reported-by: Sivasubramanian Ramasubramanian
Discussion: https://postgr.es/m/1531867212836.63354%40amazon.com
Author: Sivasubramanian Ramasubramanian, Alexander Korotkov
Backpatch-through: 9.4

Branch
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REL9_5_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/3c09b032afe548a4f0c3fd0658957650c3e22832

Modified Files
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src/backend/access/gin/gindatapage.c | 20 ++++++++++++--------
src/backend/access/gin/ginxlog.c | 27 +++++++++++++++++++++------
2 files changed, 33 insertions(+), 14 deletions(-)