Crash in IE 11

[tiblu]

COPIED OVER FROM - tiblu#5 - by @marcovtwout

When connecting to a pad (unauthenticated) in Internet Explorer 11 and typing something, etherpad crashes.

[2018-06-06 11:32:38.790] [ERROR] console - TypeError: Cannot read property 'id' of undefined
    at Object.exports.handleMessage [as hook_fn] (/opt/etherpad/etherpad-lite/node_modules/ep_webhooks/serverHooks.js:95:55)
    at normalizedhook (/opt/etherpad/etherpad-lite/src/static/js/pluginfw/hooks.js:15:27)
    at hookCallWrapper (/opt/etherpad/etherpad-lite/src/static/js/pluginfw/hooks.js:21:14)
    at /opt/etherpad/etherpad-lite/src/static/js/pluginfw/hooks.js:88:7
    at /opt/etherpad/etherpad-lite/src/node_modules/async/lib/async.js:249:17
    at /opt/etherpad/etherpad-lite/src/node_modules/async/lib/async.js:125:13
    at Array.forEach (<anonymous>)
    at _each (/opt/etherpad/etherpad-lite/src/node_modules/async/lib/async.js:46:24)
    at async.each (/opt/etherpad/etherpad-lite/src/node_modules/async/lib/async.js:124:9)
    at _asyncMap (/opt/etherpad/etherpad-lite/src/node_modules/async/lib/async.js:248:13)

user is undefined here: https://github.com/tiblu/ep_webhooks/blob/master/serverHooks.js#L95