py/vm: "yield from" didn't handle MP_OBJ_STOP_ITERATION optimization.

E.g. crashed when yielding from already stopped generators.

py/objgenerator.c

@@ -99,6 +99,7 @@ mp_vm_return_kind_t mp_obj_gen_resume(mp_obj_t self_in, mp_obj_t send_value, mp_
     assert(MP_OBJ_IS_TYPE(self_in, &mp_type_gen_instance));
     mp_obj_gen_instance_t *self = MP_OBJ_TO_PTR(self_in);
     if (self->code_state.ip == 0) {
+        // Trying to resume already stopped generator
         *ret_val = MP_OBJ_STOP_ITERATION;
         return MP_VM_RETURN_NORMAL;
     }

py/vm.c

@@ -1144,7 +1144,8 @@ yield:
                     if (ret_kind == MP_VM_RETURN_NORMAL) {
                         // Pop exhausted gen
                         sp--;
-                        if (ret_value == MP_OBJ_NULL) {
+                        // TODO: When ret_value can be MP_OBJ_NULL here??
+                        if (ret_value == MP_OBJ_NULL || ret_value == MP_OBJ_STOP_ITERATION) {
                             // Optimize StopIteration
                             // TODO: get StopIteration's value
                             PUSH(mp_const_none);