Don't pass InvalidTransactionId to update_next_xid.

update_next_xid() doesn't have any special treatment for the invalid
or other special XIDs, so it will treat InvalidTransactionId (0) as a
regular XID. If old nextXid is smaller than 2^31, 0 will look like a
very old XID, and nothing happens. But if nextXid is greater than 2^31
0 will look like a very new XID, and update_next_xid() will
incorrectly bump up nextXID.

pageserver/src/walingest.rs

@@ -102,7 +102,9 @@ impl WalIngest {
         buf.advance(decoded.main_data_offset);
 
         assert!(!self.checkpoint_modified);
-        if self.checkpoint.update_next_xid(decoded.xl_xid) {
+        if decoded.xl_xid != pg_constants::INVALID_TRANSACTION_ID
+            && self.checkpoint.update_next_xid(decoded.xl_xid)
+        {
             self.checkpoint_modified = true;
         }