nir: Handle complex derefs in nir_split_array_vars

We already bail and don't split the vars but we were passing a NULL to
_mesa_hash_table_search which is not allowed.

Fixes: f1cb3348 "nir/split_vars: Properly bail in the presence of ..."
Reviewed-by: Caio Marcelo de Oliveira Filho <caio.oliveira@intel.com>
(cherry picked from commit 37cdb7fc)