CLI/reindex: fix memory leak

CLI/reindex: fix memory leak

Since message is owned by messages, it was held for the entire run of
the program. This in turn means that the Xapian::Document objects are
not freed, and thus one ends up with (effectively) a copy of one's
entire mailstore in memory when running

       notmuch reindex '*'

Thanks to Olly Betts for the patient help debugging, and the
suggestion of a fix.