Guard against content view being null in onOverScrolled

Summary:
It seems that the content view can sometimes be null when onOverScrolled is called (presumably when the scrollview gets unmounted while it's in the middle of scrolling?).

Changelog: [Android][fixed] - Guard against content view being null in onOverScrolled.

Reviewed By: mdvacca

Differential Revision: D14737534

fbshipit-source-id: e88ec6f585e50517b734a8809fc3843c0b22df10

ReactAndroid/src/main/java/com/facebook/react/views/scroll/ReactScrollView.java

@@ -665,7 +665,7 @@ public void setEndFillColor(int color) {
 
   @Override
   protected void onOverScrolled(int scrollX, int scrollY, boolean clampedX, boolean clampedY) {
-    if (mScroller != null) {
+    if (mScroller != null && mContentView != null) {
       // FB SCROLLVIEW CHANGE
 
       // This is part two of the reimplementation of fling to fix the bounce-back bug. See #fling() for