LGTM-Alert Prototype pollution?

[Uzlopak]

Prerequisites

• I have written a descriptive issue title
• I have searched existing issues to ensure the issue has not already been raised

Issue

We should just check if this is a valid alert or not.

https://lgtm.com/projects/g/fastify/fast-json-stringify?mode=tree&ruleFocus=1513136283260

According to lgtm it was introduced with #504

[mcollina]

It's not a problem: https://github.com/fastify/fast-json-stringify#security

[climba03003]

Even the alert itself should be false positive.

fjsCloned is a symbol, not user provided string. It can never be __proto__ and trigger the problem described.