pgsql: Fix another palloc in critical section.

Fix another palloc in critical section.

Also add a regression test for a GIN index with enough items with the same
key, so that a GIN posting tree gets created. Apparently none of the
existing GIN tests were large enough for that.

This code is new, no backpatching required.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/ffbba6ee1244901b492fe268ea94f72e35aedf38

Modified Files
--------------
src/backend/access/gin/gindatapage.c | 35 ++++++++++++++++------------
src/test/regress/expected/create_index.out | 14 +++++++++++
src/test/regress/sql/create_index.sql | 14 +++++++++++
3 files changed, 48 insertions(+), 15 deletions(-)