Finding memory corruption in an extension

An extension I'm creating is causing Postgres to crash, almost certainly
due to memory corruption. I am using palloc0/pfree, calling SET_VARSIZE,
and generally following the procedures documented here:
https://www.postgresql.org/docs/12/xfunc-c.html. I am also testing my code
outside of Postgres (using alloc/free instead of palloc0/pfree), and
valgrind is not finding any corruption or leaks.

The crash is not completely reproducible, but when it does happen, it's
pretty fast -- create a table, insert a couple of rows, explain a query.
(My goal is to create a GIN index on my datatype, but this crash occurs
without the index.)

I'm interested in advice on how to go about hunting down my problem.
Something along the lines of a debugging malloc, or valgrind, for Postgres.

Jack Orenstein