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

src/backend/executor/nodeModifyTable.c

@@ -2967,21 +2967,20 @@ lmerge_matched:
 					 */
 
 					/*
-					 * If cpUpdateRetrySlot is set, ExecCrossPartitionUpdate()
-					 * must have detected that the tuple was concurrently
-					 * updated, so we restart the search for an appropriate
-					 * WHEN MATCHED clause to process the updated tuple.
+					 * During an UPDATE, if cpUpdateRetrySlot is set, then
+					 * ExecCrossPartitionUpdate() must have detected that the
+					 * tuple was concurrently updated, so we restart the
+					 * search for an appropriate WHEN MATCHED clause to
+					 * process the updated tuple.
 					 *
 					 * In this case, ExecDelete() would already have performed
 					 * EvalPlanQual() on the latest version of the tuple,
 					 * which in turn would already have been loaded into
 					 * ri_oldTupleSlot, so no need to do either of those
 					 * things.
-					 *
-					 * XXX why do we not check the WHEN NOT MATCHED list in
-					 * this case?
 					 */
-					if (!TupIsNull(context->cpUpdateRetrySlot))
+					if (commandType == CMD_UPDATE &&
+						!TupIsNull(context->cpUpdateRetrySlot))
 						goto lmerge_matched;
 
 					/*