nir/serialize: Prevent writing uninitialized state_slot data

nir/serialize: Prevent writing uninitialized state_slot data

The nir_state_slot struct had some padding that was never initialized.
Serializing the individual parts of the struct is more robust and avoids
the overhead of zeroing it at creation, so just do that.

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>