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@gmail.com>
Reviewed-by: Ewan Young <kdbase.hack@gmail.com>
Discussion: https://postgr.es/m/CALdSSPjTS6TYe5=5NfMUBYZyQu5cn=ABL6K5_OZjzGWqnwXeBw@mail.gmail.com
Backpatch-through: 18