Fix critical bug: "Plugin graph failed: TypeError: Cannot call method 'call' of undefined"

The graph plugin had several critical errors like this

2014-10-22T23:44:45.698Z [CRIT] [67CCEC68-8D7C-47B1-96F8-500775A274DA.1] [core] Plugin graph failed: TypeError: Cannot call method 'call' of undefined
    at Object.plugins.run_next_hook (/usr/lib/node_modules/Haraka/plugins.js:334:28)
    at Object.plugins.run_hooks (/usr/lib/node_modules/Haraka/plugins.js:223:13)
    at Connection.queue_respond (/usr/lib/node_modules/Haraka/connection.js:1553:25)
    at callback (/usr/lib/node_modules/Haraka/plugins.js:306:39)
    at /usr/lib/node_modules/Haraka/outbound.js:423:19
    at /usr/lib/node_modules/Haraka/node_modules/async/lib/async.js:142:25
    at /usr/lib/node_modules/Haraka/outbound.js:445:17
    at Object.oncomplete (fs.js:107:15)