Free memory used for last stripe read when re-scanning a columnar table (#5143)

Instead of setting stripeReadState to NULL, call ColumnarResetRead
before re-scanning a columnar table since this function is already
designed for doing the necessary clean up when finishing a stripe
read.

Note that this change shouldn't have a great effect on memory usage
since AdvanceStripe was already doing the clean-up for all the
stripes except the last one.

src/backend/columnar/columnar_reader.c

@@ -432,7 +432,7 @@ HasUnreadStripe(ColumnarReadState *readState)
 void
 ColumnarRescan(ColumnarReadState *readState)
 {
-	readState->stripeReadState = NULL;
+	ColumnarResetRead(readState);
 	readState->currentStripeMetadata = FindNextStripeByRowNumber(readState->relation,
 																 COLUMNAR_INVALID_ROW_NUMBER,
 																 GetTransactionSnapshot());