Fix bug in ssz decoding

eth2/utils/ssz/tests/tests.rs

@@ -229,6 +229,18 @@ mod round_trip {
         d: Vec<u16>,
     }
 
+    #[test]
+    fn three_variable_len() {
+        let vec: Vec<ThreeVariableLen> = vec![ThreeVariableLen {
+            a: 42,
+            b: vec![0],
+            c: vec![1],
+            d: vec![2],
+        }];
+
+        round_trip(vec);
+    }
+
     #[test]
     fn offsets_decreasing() {
         let bytes = vec![