Once gunzipped, this message (a bounced spam message) causes
notmuch-show-mode to crash part way through redisplay in emacs 27. It
seems fine in emacs-28.0.50 (Sept 26, after some related Gnus changes by
larsi).

It seems related to [1], but there is no crypto involved. 

[1]: id:20210703201131.3184165-1-david@tethera.net

I'm not sure if it's worth adding to the test suite, I'm posting so as
not to lose track of the issue.