uncaughtExceptions in lib/flow-event

[jrgm]

In releasing the train, I had these, so had to roll back to previous. Not terribly common, but no choice.

TypeError: Cannot read property 'domainLookupStart' of undefined
at /data/fxa-content-server/server/lib/flow-event.js:133:38
at Array.reduce (native)
at /data/fxa-content-server/server/lib/flow-event.js:132:33
at Array.forEach (native)
at module.exports.events.map.flowEvents.forEach.PERFORMANCE_TIMINGS.forEach.item (/data/fxa-content-server/server/lib/flow-event.js:131:25)
at /data/fxa-content-server/server/lib/routes/post-metrics.js:155:9
at nextTickCallbackWith0Args (node.js:489:9)
at process._tickDomainCallback (node.js:459:13)

[vladikoff]

Ref: #4776

cc @philbooth , any quick fixes for this?