pgsql: Fix gin_leafpage_items().

Fix gin_leafpage_items().

On closer inspection, commit 84ad68d64 broke gin_leafpage_items(),
because the aligned copy of the page got palloc'd in a short-lived
context whereas it needs to be in the SRF's multi_call_memory_ctx.
This was not exposed by the regression test, because the regression
test doesn't actually exercise the function in a meaningful way.
Fix the code bug, and extend the test in what I hope is a portable
fashion.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/367b99bbb13a14bd96bba6f73c231b2056ba9592

Modified Files
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contrib/pageinspect/expected/gin.out | 11 +++++++++--
contrib/pageinspect/ginfuncs.c       |  6 +++---
contrib/pageinspect/sql/gin.sql      |  9 +++++++--
3 files changed, 19 insertions(+), 7 deletions(-)