Check owning object wasn't disclaimed.

Some objects are serialized as part of an enclosing object (more for
historical reasons than good ones at this point, alas). Check we did
not clear an SC when doing repossession.

src/6model/sc.c

@@ -327,8 +327,12 @@ void MVM_sc_wb_hit_obj(MVMThreadContext *tc, MVMObject *obj) {
             MVMint64 i;
             for (i = 0; i < n; i += 2) {
                 if (MVM_repr_at_pos_o(tc, owned_objects, i) == obj) {
+                    MVMSerializationContext *real_sc;
                     obj = MVM_repr_at_pos_o(tc, owned_objects, i + 1);
-                    if (MVM_sc_get_obj_sc(tc, obj) == comp_sc)
+                    real_sc = MVM_sc_get_obj_sc(tc, obj);
+                    if (!real_sc)
+                        return; /* Probably disclaimed. */
+                    if (real_sc == comp_sc)
                         return;
                     found = 1;
                     break;