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: 9fbd390dd4 "nir: Add support for cloning shaders"
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
(cherry picked from commit 005cc9ae37)
ab75e1e289