spamassassin: check transaction before accessing

so that this error doesn't happen:

Aug 31 16:01:09 filter1-mail haraka[73661]: [CRIT] [-] [core] TypeError:
Cannot read property 'message_stream' of null
Aug 31 16:01:09 filter1-mail node[73661]: [CRIT] [-] [core]     at
Socket.<anonymous>
(/usr/local/lib/node_modules/Haraka/plugins/spamassassin.js:32:31)
Aug 31 16:01:09 filter1-mail node[73661]: [CRIT] [-] [core]     at Socket.emit
(events.js:117:20)
Aug 31 16:01:09 filter1-mail node[73661]: [CRIT] [-] [core]     at
Object.afterConnect [as oncomplete] (net.js:886:10)