Fix crash in RCTImageComponentView.didReceiveImage

Summary:
Changelog: [Internal]

I went over the ownership model in this call `_state->getData().getImageRequest().getImageInstrumentation().didSetImage();`

1. `getData()` returns reference to an object that is copied to state.
2. `getImageRequest()` returns a reference to an object that is strongly owned by `ImageState`.
3. `getImageInstrumentation()` returns a reference to an object that is strongly owned by `ImageRequest`.

I don't think any of these can cause a crash, however `_state` can be nullptr.

Reviewed By: shergin

Differential Revision: D19599258

fbshipit-source-id: 317f03cd9a53d83f64ccbb9f9abfce10fcc1fae5