Zero-initialize parent_cpstate in analyze_cypher

cypher_parsestate parent_cpstate is declared on the stack in
analyze_cypher() and only pstate, graph_name, and params are explicitly
set before it is passed to make_cypher_parsestate(). The latter reads
parent_cpstate->subquery_where_flag (and other fields) in
cypher_parse_node.c, which leaves them with indeterminate values. UBSan
flagged the garbage bool (value 8) and aborted the backend.

Use MemSet to zero the struct before populating the explicit fields so
all remaining members start with a defined value.

Author: claude

src/backend/parser/cypher_analyze.c

@@ -961,6 +961,7 @@ static Query *analyze_cypher(List *stmt, ParseState *parent_pstate,
      * convert ParseState into cypher_parsestate temporarily to pass it to
      * make_cypher_parsestate()
      */
+    MemSet(&parent_cpstate, 0, sizeof(parent_cpstate));
     parent_cpstate.pstate = *parent_pstate;
     parent_cpstate.graph_name = NULL;
     parent_cpstate.params = NULL;