HeapTupleHeaderGetCmax() asserts that it is only used if the tuple has
been updated by the current transaction. That check is correct and
sensible but requires allocating memory if xmax is a multixact. When
wal_level is set to logical cmax needs to be included in a wal record
, generated inside a critical section, which can trigger the assertion
added in 4a170ee9.

Reported-By: Steve Singer