[PATCH] Check for TupleTableSlot nullness before dereferencing

Hello everyone,

I'd like to propose adding check for nullness of
TupleTableSlot before dereferencing it in /src/backend/executor/nodeAgg.c

It is done in the same manner other TupleTableSlots are checked,
but was probably left unseen because slot1 and slot2 variables
can be swapped during function execution.

The patch is attached.

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Best regards,
Alexander Kuznetsov