fix the crash caused by nil partialLoadHandler
fix the crash caused by nil partialLoadHandler

Summary:
## Problem
the partialLoadHandler is nil on line 338 of RCTImageLoader, therefore, if there is a cached image, it would crash on line 495.

## Change
Check if partialLoadHandler is nil on line 495 to prevent the crash

Changelog: [iOS][Changed] - fix the crash caused by nil partialLoadHandler

Reviewed By: appden

Differential Revision: D34544090

fbshipit-source-id: f9965700e529c5add1e25867a3772c053447d99a

janicduplessis/react-native,facebook/react-native,facebook/react-native,javache/react-native,janicduplessis/react-native,janicduplessis/react-native,javache/react-native,javache/react-native,facebook/react-native,javache/react-native,javache/react-native,facebook/react-native,javache/react-native,javache/react-native,javache/react-native,facebook/react-native,facebook/react-native,janicduplessis/react-native,janicduplessis/react-native,facebook/react-native,javache/react-native,janicduplessis/react-native,janicduplessis/react-native,janicduplessis/react-native,facebook/react-native,facebook/react-native