BF assertion failure on mandrill in walsender, v13

Hi,

Not sure if there is much chance of debugging this one-off failure in
without a backtrace (long shot: any chance there's still a core
file?), but for the record: mandrill choked on a null pointer passed
to GetMemoryChunkContext() inside a walsender running logical
replication.  Possibly via pfree(NULL), but there are other paths.
That's an animal running with force_parallel_mode and
RANDOMIZE_ALLOCATED_MEMORY, on AIX with IBM compiler in 32 bit mode,
so unusual in several ways.

https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=mandrill&dt=2021-06-06%2015:37:23