Avoid error if _snakeForward if polyline is not on the map

[YuanxiangFranck]

When Polyline is removed before the animation is finished the following error is thrown:
Uncaught TypeError: Cannot read property 'latLngToContainerPoint' of null

[IvanSanchez]

Actually, this should be if (!this._map), for consistency with other Leaflet code. You cannot be sure that this._map will be either null or undefined when the layer is removed, just that it will be falsy.

[YuanxiangFranck]

Well this._map == null handle both null and undefined
In general it's safer to use this because it can return false if this._map = false

But here I don't think this._map can be either 0 or false, so if you prefer to keep consistency I can change it.

[IvanSanchez]

I know that == will work, but in Leaflet we try to avoid using == and using === instead.

For the style that is currently used in Leaflet, see https://github.com/Leaflet/Leaflet/blob/5cc9b79c97be3df653474a774f3b06af252a44bc/src/layer/ImageOverlay.js#L96 and https://github.com/Leaflet/Leaflet/blob/faeb46b27ccba0f3691ad2a074a46ae3be55538c/src/control/Control.js#L81 , for example. The code just checks for truthy/falsy.

So I would prefer you to change it. It's not a big problem, but just a small coding style issue.