pgsql: Fix memory leak in Memoize code

Fix memory leak in Memoize code

Ensure we switch to the per-tuple memory context to prevent any memory
leaks of detoasted Datums in MemoizeHash_hash() and MemoizeHash_equal().

Reported-by: Orlov Aleksej
Author: Orlov Aleksej, David Rowley
Discussion: https://postgr.es/m/83281eed63c74e4f940317186372abfd%40cft.ru
Backpatch-through: 14, where Memoize was added

Branch
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REL_15_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/689af6db6c76a7966efdb702326f85fa0d8a04ef

Modified Files
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src/backend/executor/nodeMemoize.c | 25 ++++++++++++++++++++++---
1 file changed, 22 insertions(+), 3 deletions(-)