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.

notmuch-reindex.c

@@ -71,6 +71,7 @@ reindex_query (notmuch_database_t *notmuch, const char *query_string,
 	ret = notmuch_message_reindex(message, indexopts);
 	if (ret != NOTMUCH_STATUS_SUCCESS)
 	    break;
+	notmuch_message_destroy (message);
     }
 
     if (!ret)