nir: Fix clone of nir_variable state slots

When num_state_slots is 0, don't create the array.  This was
triggering the following assert when running vkcube with
NIR_TEST_CLONE=1

    vkcube: ../src/compiler/nir/nir_split_per_member_structs.c:66:
    split_variable: Assertion `var->state_slots == NULL' failed.

Fixes: 9fbd390dd4b "nir: Add support for cloning shaders"
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
(cherry picked from commit 005cc9ae37ca45960d87389dc9eace5ed29d1b99)