Deleting a matched relationship and then executing full-path `MERGE` causes a PostgreSQL backend segmentation fault

[YGY-001]

Docker image apache/age:release_PG18_1.7.0

Reproduction

LOAD 'age';
SET search_path = ag_catalog, public;
SELECT ag_catalog.create_graph('age_issue_case11');

SELECT *
FROM ag_catalog.cypher('age_issue_case11', $$
CREATE (a:base {id: 1})-[r1:r {k0: false}]->
       (n:base {id: 2})-[r0:s {k5: 'x'}]->(b:base {id: 3})
$$) AS (created agtype);

SELECT *
FROM ag_catalog.cypher('age_issue_case11', $$
MATCH (a:base)-[r1:r]->(n:base)-[r0:s]->(b:base)
SET r0.k5 = 'J', r1.k0 = true
WITH range(0, 0) + [0] AS keep, r1 AS old
WHERE 0 IN keep
DELETE old
CREATE (:x)
MERGE p0 = ({id: 129})-[r2:r]->(n2 {id: 130})-[:s]->(:c {id: 131})
RETURN 1
$$) AS (value agtype);

Expected

The query returns one row containing 1 and PostgreSQL remains available.

Actual

server closed the connection unexpectedly
client backend ... was terminated by signal 11: Segmentation fault

[gregfelice]

Confirmed still crashing on current master (4d9b2506, AGE 1.8.0, PostgreSQL 18.4).

Backtrace at the fault:

#0  0x00007fc0a258ae15 in apply_update_list (node=node@entry=0x55a1935f0ac8, set_info=0x55a1935f0cf8) at src/backend/executor/cypher_set.c:420
#1  0x00007fc0a258bb9c in process_update_list (node=0x55a1935f0ac8) at src/backend/executor/cypher_set.c:866
#2  exec_cypher_set (node=0x55a1935f0ac8) at src/backend/executor/cypher_set.c:930
#3  0x000055a181ffd032 in ?? ()

This faults in apply_update_list at src/backend/executor/cypher_set.c:420, below process_update_list (:866) and exec_cypher_set (:930) — the SET executor. Distinct from the create_vertex group, though the query shape mixes SET, DELETE, CREATE and MERGE in one statement.

For context: I triaged the current fuzz-report backlog against master today. There are six distinct crash sites in it, not one — the largest group is five issues (#2501, #2508, #2527, #2530, #2535) that all share a single NULL dereference in create_vertex, written up in #2535. This issue is not part of that group; it is its own defect and needs its own fix.