pgsql: amcheck: Fix memory leak with gin_index_check()

amcheck: Fix memory leak with gin_index_check()

"prev_tuple" was overwritten with a new tuple coming from
CopyIndexTuple() on each loop, leaking memory for every tuple processed
on entry tree pages. The function uses a dedicated memory context, but
this could leave unused large areas of memory while processing a large
GIN index, the larger the worse.

Oversight in 14ffaece0fb5.

Author: Kirill Reshke <reshkekirill(at)gmail(dot)com>
Reviewed-by: Ewan Young <kdbase(dot)hack(at)gmail(dot)com>
Discussion: https://postgr.es/m/CALdSSPjTS6TYe5=5NfMUBYZyQu5cn=ABL6K5_OZjzGWqnwXeBw@mail.gmail.com
Backpatch-through: 18

Branch
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REL_19_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/80cfd8aef645295aceafd2aa98912c64edd06394

Modified Files
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contrib/amcheck/verify_gin.c | 3 +++
1 file changed, 3 insertions(+)