pgsql: Validate ltree siglen GiST option to be int-aligned

Validate ltree siglen GiST option to be int-aligned

Unaligned siglen could lead to an unaligned access to subsequent key fields.

Backpatch to 13, where opclass options were introduced.

Reported-by: Alexander Lakhin
Bug: 17847
Discussion: https://postgr.es/m/17847-171232970bea406b%40postgresql.org
Reviewed-by: Tom Lane, Pavel Borisov, Alexander Lakhin
Backpatch-through: 13

Branch
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REL_14_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/f4a4a18ecbdb74da70679c7bcaa395f66b68c4af

Modified Files
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contrib/ltree/expected/ltree.out |  9 +++++++--
contrib/ltree/ltree_gist.c       | 17 ++++++++++++++++-
contrib/ltree/sql/ltree.sql      |  2 ++
doc/src/sgml/ltree.sgml          |  3 ++-
4 files changed, 27 insertions(+), 4 deletions(-)