Fabric: Fixing crash in RCTScrollViewComponentView (2nd attempt)

Summary:
Seems a ScrollView sometimes calls the delegate in own destructor; and seems that in some configurations the delegate is also already destroyed at this point. I am not sure if this a bug in UIKit or not, but seems the fix is easy, we just have to clear the ScrollView's delegate on the delegate's deallocation.
This issue is also looks similar:
https://stackoverflow.com/questions/18778691/crash-on-exc-breakpoint-scroll-view/19011871

Changelog: [iOS] [Fixed] - Fixed crash in RCTScrollViewComponentView

Reviewed By: sammy-SC

Differential Revision: D17924429

fbshipit-source-id: 5727bca9f028e28f76f60304c187ee39eb6e1856

React/Fabric/Mounting/ComponentViews/ScrollView/RCTScrollViewComponentView.mm

@@ -85,7 +85,8 @@ - (instancetype)initWithFrame:(CGRect)frame
 
 - (void)dealloc
 {
-  [_scrollViewDelegateSplitter removeDelegate:self];
+  // This is not strictly necessary but that prevents a crash caused by a bug in UIKit.
+  _scrollView.delegate = nil;
 }
 
 #pragma mark - RCTComponentViewProtocol