Invalid JSON logged when toString function returns undefined

[NiXXeD]

Similar to #456

We found a situation where a some errors we were logging had the output contain "toString":[object Undefined]. This isn't valid JSON and was causing our log aggregator parser to break.

In playing with it, I am able to reproduce this by:

const pino = require('pino')()
const err = new Error('Blah')
err.toString = () => undefined
pino.error(err, 'Something bad happened.')

The full output then is this, which is not valid JSON.

{
  "level": "error",
  "time": 1538513586038,
  "msg": "Something bad happened.",
  "pid": 4086,
  "hostname": "dev.local",
  "type": "Error",
  "stack": "...",
  "toString": [object Undefined],
  "v": 1
}

I'm guessing this isn't exactly how it's produced, but this is a way to reproduce the same situation.

[NiXXeD]

Yup this is fixed as of 5.6.3. Thank you!

[github-actions]

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