Fix use of wrong index in ComputeXidHorizons().

This bug, recently introduced in 941697c3, at least lead to vacuum
failing because it found tuples inserted by a running transaction, but
below the freeze limit. The freeze limit in turn is directly affected
by the aforementioned bug.

Thanks to Tom Lane figuring how to make the bug reproducible.

We should add a few more assertions to make sure this type of bug
isn't as hard to notice, but it's not yet clear how to best do so.
Co-Diagnosed-By: Tom Lane <tgl@sss.pgh.pa.us>
Author: Andres Freund <andres@anarazel.de>
Discussion: https://postgr.es/m/1013484.1597609043@sss.pgh.pa.us

src/backend/storage/ipc/procarray.c

@@ -1663,7 +1663,7 @@ ComputeXidHorizons(ComputeXidHorizonsResult *h)
 		TransactionId xmin;
 
 		/* Fetch xid just once - see GetNewTransactionId */
-		xid = UINT32_ACCESS_ONCE(other_xids[pgprocno]);
+		xid = UINT32_ACCESS_ONCE(other_xids[index]);
 		xmin = UINT32_ACCESS_ONCE(proc->xmin);
 
 		/*