Fix Coverity issue reported in commit 2273fa32bc.

Coverity complains that the return value from gettuple_eval_partition
(stored in variable "datum") in a do..while loop in
WinGetFuncArgInPartition is overwritten when exiting the while
loop. This commit tries to fix the issue by changing the
gettuple_eval_partition call to:

(void) gettuple_eval_partition()

explicitly stating that we discard the return value. We are just
interested in whether we are inside or outside of partition, NULL or
NOT NULL here.

Also enhance some comments for easier code reading.

Reported-by: Michael Paquier <michael@paquier.xyz>
Discussion: https://postgr.es/m/aPCOabSE4VfJLaky%40paquier.xyz

src/backend/executor/nodeWindowAgg.c

@@ -3270,8 +3270,9 @@ window_gettupleslot(WindowObject winobj, int64 pos, TupleTableSlot *slot)
 	return true;
 }
 
-/*
+/* gettuple_eval_partition
- * get tuple and evaluate in partition
+ * get tuple in a patition and evaluate the window function's argument
+ * expression on it.
  */
 static Datum
 gettuple_eval_partition(WindowObject winobj, int argno,
@@ -3790,8 +3791,14 @@ WinGetFuncArgInPartition(WindowObject winobj, int argno,
 			continue;			/* keep on moving forward or backward */
 		else					/* need to check NULL or not */
 		{
-			/* get tuple and evaluate in partition */
+			/*
-			datum = gettuple_eval_partition(winobj, argno,
+			 * NOT NULL info does not exist yet.  Get tuple and evaluate func
+			 * arg in partition. We ignore the return value from
+			 * gettuple_eval_partition because we are just interested in
+			 * whether we are inside or outside of partition, NULL or NOT
+			 * NULL.
+			 */
+			(void) gettuple_eval_partition(winobj, argno,
 										   abs_pos, isnull, &myisout);
 			if (myisout)		/* out of partition? */
 				break;
@@ -3802,7 +3809,7 @@ WinGetFuncArgInPartition(WindowObject winobj, int argno,
 		}
 	} while (notnull_offset < notnull_relpos);
 
-	/* get tuple and evaluate in partition */
+	/* get tuple and evaluate func arg in partition */
 	datum = gettuple_eval_partition(winobj, argno,
 									abs_pos, isnull, &myisout);
 	if (!myisout && set_mark)