Fix calculation of maximal multixact in ingest_multixact_create_record (#6502)

## Problem

See https://neondb.slack.com/archives/C06F5UJH601/p1706373716661439

## Summary of changes

Use None instead of 0 as initial accumulator value for calculating
maximal multixact XID.

## Checklist before requesting a review

- [ ] I have performed a self-review of my code.
- [ ] If it is a core feature, I have added thorough tests.
- [ ] Do we need to implement analytics? if so did you add the relevant
metrics to the dashboard?
- [ ] If this PR requires public announcement, mark it with
/release-notes label and add several sentences in this section.

## Checklist before merging

- [ ] Do not forget to reformat commit message to not include the above
checklist

---------

Co-authored-by: Konstantin Knizhnik <knizhnik@neon.tech>
Co-authored-by: Heikki Linnakangas <heikki@neon.tech>

pageserver/src/walingest.rs

@@ -1363,16 +1363,22 @@ impl WalIngest {
             self.checkpoint.nextMultiOffset = xlrec.moff + xlrec.nmembers;
             self.checkpoint_modified = true;
         }
-        let max_mbr_xid = xlrec.members.iter().fold(0u32, |acc, mbr| {
-            if mbr.xid.wrapping_sub(acc) as i32 > 0 {
-                mbr.xid
+        let max_mbr_xid = xlrec.members.iter().fold(None, |acc, mbr| {
+            if let Some(max_xid) = acc {
+                if mbr.xid.wrapping_sub(max_xid) as i32 > 0 {
+                    Some(mbr.xid)
+                } else {
+                    acc
+                }
             } else {
-                acc
+                Some(mbr.xid)
             }
         });
 
-        if self.checkpoint.update_next_xid(max_mbr_xid) {
-            self.checkpoint_modified = true;
+        if let Some(max_xid) = max_mbr_xid {
+            if self.checkpoint.update_next_xid(max_xid) {
+                self.checkpoint_modified = true;
+            }
         }
         Ok(())
     }

test_runner/regress/test_next_xid.py

@@ -203,6 +203,16 @@ def test_import_at_2bil(
         $$;
         """
     )
+
+    # Also create a multi-XID with members past the 2 billion mark
+    conn2 = endpoint.connect()
+    cur2 = conn2.cursor()
+    cur.execute("INSERT INTO t VALUES ('x')")
+    cur.execute("BEGIN; select * from t WHERE t = 'x' FOR SHARE;")
+    cur2.execute("BEGIN; select * from t WHERE t = 'x' FOR SHARE;")
+    cur.execute("COMMIT")
+    cur2.execute("COMMIT")
+
     # A checkpoint writes a WAL record with xl_xid=0. Many other WAL
     # records would have the same effect.
     cur.execute("checkpoint")
@@ -217,4 +227,4 @@ def test_import_at_2bil(
     conn = endpoint.connect()
     cur = conn.cursor()
     cur.execute("SELECT count(*) from t")
-    assert cur.fetchone() == (10000 + 1,)
+    assert cur.fetchone() == (10000 + 1 + 1,)