pgsql: Don't rely on uninitialized value in MERGE / DELETE

Don't rely on uninitialized value in MERGE / DELETE

On MERGE / WHEN MATCHED DELETE it's not possible to get cross-partition
updates, so we don't initialize cpUpdateRetrySlot; however, the code was
not careful to ignore the value in that case.  Make it do so.

Backpatch to 15.

Reported-by: Alexander Lakhin <exclusion@gmail.com>
Reviewed-by: Dean Rasheed <dean.a.rasheed@gmail.com>
Discussion: https://postgr.es/m/17792-0f89452029662c36@postgresql.org

Branch
------
master

Details
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https://git.postgresql.org/pg/commitdiff/fd923b5de94cdd3a7d05dcb843667dc228ea7fa3

Modified Files
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src/backend/executor/nodeModifyTable.c | 15 +++++++--------
1 file changed, 7 insertions(+), 8 deletions(-)