Fix timeout bug in helo.checks.js

Missing code in timeout caused the following:

The timeout block ran and returned cb(err) which in turn ran next(); 
[helo.checks] forward_dns(Error: timeout resolving: amazon.kwiveolicensings.com.)

Later - the actual DNS result came back with it's own timeout; however because there was no boolean to say that the timeout block had run:
[helo.checks] forward_dns(Error: queryA ETIMEOUT)

This caused next() to be run again:
[core] helo.checks plugin ran callback multiple times - ignoring subsequent calls
[core] Error#012    at callback (/usr/lib/node_modules/Haraka/plugins.js:272:34)#012    at cb (/usr/lib/node_modules/Haraka/plugins/helo.checks.js:427:20)#012    at /usr/lib/node_modules/Haraka/plugins/helo.checks.js:593:25#012    at asyncCallback (dns.js:68:16)#012    at Object.onanswer [as oncomplete] (dns.js:149:9)