Fix: Added scroll Bounds check in scrollToOffset method in RCTScrollView on iOS

Summary:
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The scrollToOffset method of RCTScrollView for iOS does not include bound check for the scroll offset provided to the method. This can cause the whole view to scroll out of screen if the offset provided is greater than the bounds of the View.
The same does not happen on Android, where the scroll halts once the last item of the scrollView is in the viewport.
I have added bounds check so the offset resets to the MaxOffset which makes sure the view does not scroll out of the viewport.

The issue can be observed in the following snack:
https://snack.expo.io/H1363Uo8f

![ezgif com-optimize 1](https://user-images.githubusercontent.com/16662518/36068270-2437ae88-0ef7-11e8-96dd-819b4ae0fd67.gif)

To test my changes I ran the code provided in the snack above with the react-native dependency pointing to my branch. As can be see in the video attached below, the scroll halts once it hits the end of the ScrollView even if the scroll offset provided is higher than the bounds of the ScrollView. It also does not scroll up for negative scroll offset.

![ezgif com-optimize](https://user-images.githubusercontent.com/16662518/36068130-9ae4f918-0ef3-11e8-8728-af7b2888bdb8.gif)

[IOS] [BUGFIX] [ScrollView] - Added bounds check to prevent ScrollView from scrolling to an offset which is out of bounds of the ScrollView for iOS.
Closes facebook#17927

Differential Revision: D7014466

Pulled By: shergin

fbshipit-source-id: a817738d08e1057a4c70f43373132f88fa1461c4

Author: siddhantsoni

React/Views/ScrollView/RCTScrollView.m

@@ -575,6 +575,9 @@ - (void)scrollToOffset:(CGPoint)offset
 - (void)scrollToOffset:(CGPoint)offset animated:(BOOL)animated
 {
   if (!CGPointEqualToPoint(_scrollView.contentOffset, offset)) {
+    CGFloat maxOffsetX = _scrollView.contentSize.width - _scrollView.bounds.size.width + _scrollView.contentInset.right;
+    CGFloat maxOffsetY = _scrollView.contentSize.height - _scrollView.bounds.size.height + _scrollView.contentInset.bottom;
+    offset = CGPointMake(MAX(0, MIN(maxOffsetX, offset.x)), MAX(0, MIN(maxOffsetY, offset.y)));
     // Ensure at least one scroll event will fire
     _allowNextScrollNoMatterWhat = YES;
     [_scrollView setContentOffset:offset animated:animated];