Fix for consume_xids advancing XIDs incorrectly

Hi,

I found that the consume_xids function incorrectly advances XIDs as
shown:

postgres=# select txid_current();
txid_current
--------------
746
(1 row)

postgres=# select consume_xids('100');
consume_xids
--------------
847
(1 row)

In the example, the consume_xids function consumes 100 XIDs when XID =
746, so the desired outcome from consume_xids should be 746 + 100 = 846,
which differs from the actual outcome, 847.

Behavior inside a transaction block:

postgres=# select txid_current();
txid_current
--------------
1410
(1 row)

postgres=# begin;
BEGIN
postgres=*# select consume_xids('100');
consume_xids
--------------
1511
(1 row)
postgres=*# select consume_xids('100');
consume_xids
--------------
1521
(1 row)

Here, the first call inside the transaction block consumes 100+1 XIDs
(incorrect), while the second call consumes exactly 100 XIDs (as
expected)

Summary:

The function performs incorrectly when:
- Outside of a transaction block
- The first call inside a transaction block
But works correctly when:
- After the second call inside a transaction block

The issue arises because consume_xids does not correctly count the
consumed XIDs when it calls the GetTopTransactionId function, which
allocates a new XID when none has been assigned.

With the attached patch, the consume_xids works as expected, both inside
and outside of transaction blocks as shown below:

postgres=# select txid_current();
txid_current
--------------
1546
(1 row)

postgres=# select consume_xids('100');
consume_xids
--------------
1646
(1 row)

postgres=# begin;
BEGIN
postgres=*# select consume_xids('100');
consume_xids
--------------
1746
(1 row)

postgres=*# select consume_xids('100');
consume_xids
--------------
1846
(1 row)

Regards,
Yushi