[ios] Fix crash in NTP's most visited when removing all sites.

This crash only affects pre-ui-refresh, but it is safe to add
this fix for all experiments.

Bug: 881229
Cq-Include-Trybots: luci.chromium.try:ios-simulator-cronet;luci.chromium.try:ios-simulator-full-configs
Change-Id: Ic200f8dfac105b73fe8903b050249683d1f2d1c1
Reviewed-on: https://chromium-review.googlesource.com/1210162
Commit-Queue: Justin Cohen <justincohen@chromium.org>
Reviewed-by: Mark Cogan <marq@chromium.org>
Reviewed-by: Rohit Rao <rohitrao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#589183}

ios/chrome/browser/ui/content_suggestions/content_suggestions_collection_updater.mm

@@ -365,7 +365,9 @@ NSString* const kContentSuggestionsCollectionUpdaterSnackbarCategory =
                                   completion:nil];
                    }];
 
-  // Make sure we get the right index for the section.
+  // Make sure the section is still in the model and that the index is correct.
+  if (![model hasSectionForSectionIdentifier:sectionIdentifier])
+    return;
   section = [model sectionForSectionIdentifier:sectionIdentifier];
 
   [self.collectionViewController.collectionView