pgsql: pageinspect: Fix crash with gist_page_items()

pageinspect: Fix crash with gist_page_items()

Attempting to use this function with a raw page not coming from a GiST
index would cause a crash, as it was missing the same sanity checks as
gist_page_items_bytea(). This slightly refactors the code so as all the
basic validation checks for GiST pages are done in a single routine,
in the same fashion as the pageinspect functions for hash and BRIN.

This fixes an issue similar to 076f4d9. A test is added to stress for
this case. While on it, I have added a similar test for
brin_page_items() with a combination make of a valid GiST index and a
raw btree page. This one was already protected, but it was not tested.

Reported-by: Egor Chindyaskin
Author: Dmitry Koval
Discussion: https://postgr.es/m/17815-fc4a2d3b74705703@postgresql.org
Backpatch-through: 14

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/be753639d35df72c1a7b42ec114393fd962f2b01

Modified Files
--------------
contrib/pageinspect/expected/brin.out | 8 ++--
contrib/pageinspect/expected/gist.out | 10 +++--
contrib/pageinspect/gistfuncs.c | 82 +++++++++++++++++------------------
contrib/pageinspect/sql/brin.sql | 8 ++--
contrib/pageinspect/sql/gist.sql | 10 +++--
5 files changed, 62 insertions(+), 56 deletions(-)