fix: add isNaN check for concurrency limit

[WaleedAshraf]

Closes #788

[ankon]

Out of interest: How did you manage to get this to be NaN in the first place? I thinking there's probably more places that don't specifically look at NaN in the code base because there's no reasonable path to get into that state.

[Nevon]

I'm generally not a big fan of these runtime type checks when there isn't a reasonable way that you'd expect to possibly receive weird values, but given that it's already checking that the limit is a number I supposed there's no harm in also checking that it's not NaN (love that NaN is a number and also neither equal to, larger or smaller than 1). But yeah, how did you get it to be NaN to begin with? 😃

[WaleedAshraf]

Before we pass config values to kafkajs, we do parseInt. And, There is WTF JS 😆

Number.parseInt(undefined) // NaN