[110][IOS] Check RLEntry when showing context menu

It is possible that `ReadingListMediator entryFromItem:`
returns nullptr, so we must check for this before
dereferencing the pointer.

(cherry picked from commit 84e06b7)

Bug: 1401484
Change-Id: I150c3b38b2e869952f1405669f174489b0de87fe
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4118475
Reviewed-by: David Jean <[email protected]>
Commit-Queue: David Jean <[email protected]>
Auto-Submit: Olivier Robin <[email protected]>
Cr-Original-Commit-Position: refs/heads/main@{#1085836}
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4120308
Commit-Queue: Olivier Robin <[email protected]>
Cr-Commit-Position: refs/branch-heads/5481@{#47}
Cr-Branched-From: 130f3e4-refs/heads/main@{#1084008}

ios/chrome/browser/ui/reading_list/reading_list_coordinator.mm

@@ -453,7 +453,7 @@ - (UIContextMenuConfiguration*)contextMenuConfigurationForItem:
         [menuElements addObject:openInNewIncognitoTab];
 
         const ReadingListEntry* entry = [self.mediator entryFromItem:item];
-        if (entry->DistilledState() == ReadingListEntry::PROCESSED) {
+        if (entry && entry->DistilledState() == ReadingListEntry::PROCESSED) {
           [menuElements
               addObject:[actionFactory
                             actionToOpenOfflineVersionInNewTabWithBlock:^{