Fabric: Fixed crash in colorComponentsFromColor()

Summary:
This fixes a recently introduced crash in `colorComponentsFromColor()` (iOS implementation) caused by dereferencing a null pointer.

The fix is just a copy of a code fragment from a previous implementation.

Reviewed By: fkgozali

Differential Revision: D23944812

fbshipit-source-id: 977135dd75c4375affddfd75183e4890618ae819

ReactCommon/react/renderer/graphics/platform/ios/Color.cpp

@@ -21,6 +21,12 @@ SharedColor colorFromComponents(ColorComponents components) {
 }
 
 ColorComponents colorComponentsFromColor(SharedColor sharedColor) {
+  if (!sharedColor) {
+    // Empty color object can be considered as `clear` (black, fully
+    // transparent) color.
+    return ColorComponents{0, 0, 0, 0};
+  }
+
   float ratio = 256;
   Color color = *sharedColor;
   return ColorComponents{(float)((color >> 16) & 0xff) / ratio,