smack: temporarily disable layer asserts for reads

The access checks for reads are currently in the wrong spot -- see the
TODO right above the call site.

Before the read checks are corrected, remove the assert, since the layer
value might not be initialized.
demo: labels must be set before calling buxton_client_set_value

A default label ("_") is set by cli_set_value(), so if we're not setting
values through the CLI, they must be appropriately initialized by the
client.