Avoid errors in rmailsum for messages without "From"
* lisp/mail/rmailsum.el (rmail-header-summary): Be defensive about the presence of the "From" header. (Bug#49770) Copyright-paperwork-exempt: yes
* lisp/mail/rmailsum.el (rmail-header-summary): Be defensive about the presence of the "From" header. (Bug#49770) Copyright-paperwork-exempt: yes