nqp::iscont should not blow up over a null value.

This fixes a broken case of .clone that the subscripting changes in
Rakudo hit. Thus, this fixes a more general issue.

src/vm/jvm/runtime/org/perl6/nqp/runtime/Ops.java

@@ -2573,7 +2573,7 @@ public static SixModelObject setcontspec(SixModelObject obj, String confname, Si
         return obj;
     }
     public static long iscont(SixModelObject obj) {
-        return obj.st.ContainerSpec == null ? 0 : 1;
+        return obj == null || obj.st.ContainerSpec == null ? 0 : 1;
     }
     public static SixModelObject decont(SixModelObject obj, ThreadContext tc) {
         if (obj == null)