Fix crash in slider with custom image

Summary:
Changelog: [Internal]

# Why was it crashing?
Crash was caused by `data.getTrackImageRequest()` returning NULL. The crash happened inside `getCoordinator` method. If you look at the implementation of the method below

```
if (request) {
    return &request->getObserverCoordinator();
} else {
    return nullptr;
}
```

you might notice that we check for NULL, why is it crashing then? And why is it only crashing in production?

The reason is compiler's optimiser, which looks at `data.getTrackImageRequest()`, this method returns `ImageRequest const &` and sees that this method will always return a value, never NULL. Compiler then looks at our `getCoordinator` function, sees that we check there for nullity but since it can never be null, removes that branch.

# Solution
Create new SliderState with non null image observer.

Reviewed By: shergin

Differential Revision: D20491367

fbshipit-source-id: 17b7cf31feabbe6f8ece324a3d329902b2ef6a2d

ReactCommon/fabric/components/slider/SliderShadowNode.h

@@ -38,6 +38,21 @@ class SliderShadowNode final : public ConcreteViewShadowNode<
   void setSliderMeasurementsManager(
       const std::shared_ptr<SliderMeasurementsManager> &measurementsManager);
 
+  static SliderState initialStateData(
+      ShadowNodeFragment const &fragment,
+      SurfaceId const surfaceId,
+      ComponentDescriptor const &componentDescriptor) {
+    auto imageSource = ImageSource{ImageSource::Type::Invalid};
+    return {imageSource,
+            {imageSource, nullptr},
+            imageSource,
+            {imageSource, nullptr},
+            imageSource,
+            {imageSource, nullptr},
+            imageSource,
+            {imageSource, nullptr}};
+  }
+
 #pragma mark - LayoutableShadowNode
 
   Size measure(LayoutConstraints layoutConstraints) const override;