Use correct datatype for xmin variables in slot.c

Two variables storing a slot's effective_xmin and effective_catalog_xmin
were saved as XLogRecPtr, which is incorrect as these should be
TransactionIds.

Oversight in 818fefd.

Author: Bharath Rupireddy
Discussion: https://postgr.es/m/CALj2ACVPSB74mrDTFezz-LV3Oi6F3SN71QA0oUHvndzi5dwTNg@mail.gmail.com
Backpatch-through: 16

Author: michaelpq

src/backend/replication/slot.c

@@ -1545,8 +1545,8 @@ InvalidatePossiblyObsoleteSlot(ReplicationSlotInvalidationCause cause,
 	int			last_signaled_pid = 0;
 	bool		released_lock = false;
 	bool		terminated = false;
-	XLogRecPtr	initial_effective_xmin = InvalidXLogRecPtr;
-	XLogRecPtr	initial_catalog_effective_xmin = InvalidXLogRecPtr;
+	TransactionId initial_effective_xmin = InvalidTransactionId;
+	TransactionId initial_catalog_effective_xmin = InvalidTransactionId;
 	XLogRecPtr	initial_restart_lsn = InvalidXLogRecPtr;
 	ReplicationSlotInvalidationCause invalidation_cause_prev PG_USED_FOR_ASSERTS_ONLY = RS_INVAL_NONE;