fix(sdk): Fix a substract with overflow in LinkedChunk.

crates/matrix-sdk/src/event_cache/linked_chunk/mod.rs

@@ -1029,7 +1029,7 @@ impl<const CAPACITY: usize, Item, Gap> Chunk<CAPACITY, Item, Gap> {
 
         match &self.content {
             ChunkContent::Gap(..) => Position(identifier, 0),
-            ChunkContent::Items(items) => Position(identifier, items.len() - 1),
+            ChunkContent::Items(items) => Position(identifier, items.len().saturating_sub(1)),
         }
     }
 
@@ -1606,6 +1606,14 @@ mod tests {
         assert_matches!(iterator.next(), None);
     }
 
+    #[test]
+    fn test_ritems_empty() {
+        let linked_chunk = LinkedChunk::<2, char, ()>::new();
+        let mut iterator = linked_chunk.ritems();
+
+        assert_matches!(iterator.next(), None);
+    }
+
     #[test]
     fn test_items() {
         let mut linked_chunk = LinkedChunk::<2, char, ()>::new();
@@ -1623,6 +1631,14 @@ mod tests {
         assert_matches!(iterator.next(), None);
     }
 
+    #[test]
+    fn test_items_empty() {
+        let linked_chunk = LinkedChunk::<2, char, ()>::new();
+        let mut iterator = linked_chunk.items();
+
+        assert_matches!(iterator.next(), None);
+    }
+
     #[test]
     fn test_ritems_from() -> Result<(), Error> {
         let mut linked_chunk = LinkedChunk::<2, char, ()>::new();