free(): invalid size in frm

I am seeing an intermittent problem with frm in mailutils 3.19, 3.20,
and also in current git master. I don't have a reliable way to reproduce
this yet, as I am seeing different output at different times when the
mailbox has not changed.

Sometimes, this "free(): invalid size" error occurs:

$ frm -t ~/Mail/inbox.mbox
<outputs correct subject lines>
free(): invalid size
Aborted                    frm -t Mail/inbox.mbox


I will update this thread when I have further info.