lib: Audit calls to notmuch_message_get_header to handle NULL return

lib: Audit calls to notmuch_message_get_header to handle NULL return

Sebastian Spaeth reported [*] a segfault within libnotmuch when
running notmuch operations while an asyncronous offlineimap job had
removed some files from the mail store. Avoid this by handling all
cases where notmuch_message_get_header could return NULL.

[*] See message id:87d3xqti3o.fsf@SSpaeth.de on notmuch@notmuchmail.org