mpi/src/starpu_mpi.c: check the data interface defines a unpack function before calling it

mpi/src/starpu_mpi.c

@@ -933,6 +933,7 @@ static void _starpu_mpi_copy_cb(void* arg)
 		/* Data has been received as a raw memory, it has to be unpacked */
 		struct starpu_data_interface_ops *itf_src = starpu_data_get_interface_ops(args->copy_handle);
 		struct starpu_data_interface_ops *itf_dst = starpu_data_get_interface_ops(args->data_handle);
+		STARPU_ASSERT_MSG(itf_dst->unpack_data, "The data interface does not define an unpack function\n");
 		itf_dst->unpack_data(args->data_handle, 0, args->buffer, itf_src->get_size(args->copy_handle));
 		free(args->buffer);
 	}