On a host running Debian Stable with python 3.9.2, and notmuch 0.31.4,
both of the attached programs work fine. On my developement host running
notmuch 0.34, both segfault when trying to fetch the 'to' header. This
invokes a more complicated code path, which seems to have broken
sometime between 0.31.4 and 0.34.

I guess something changed in the library that the bindings need to catch
up with.