Fix : null pointer crash

[seker]

the 'target_node' of Animation Channel may be nullpointer.

[seker]

For example:

"animations": [
    {
        "channels": [
            {
                "sampler": 0,
                "target": {
                    "extensions": {
                        "KHR_animation_pointer": {
                            "pointer": "/materials/65/emissiveFactor"
                        }
                    },
                    "path": "pointer"
                }
            },
            {
                "sampler": 1,
                "target": {
                    "extensions": {
                        "KHR_animation_pointer": {
                            "pointer": "/materials/43/emissiveFactor"
                        }
                    },
                    "path": "pointer"
                }
            }
        ],