nir/print: fix coverity error

nir/print: fix coverity error

Not something actually hit in real life (now state is never non-null,
but only case state->syms is null is if nir_print_instr() path).  But it
was something I overlooked the first time, so might as well fix it.

    *** CID 1324642:  Null pointer dereferences  (REVERSE_INULL)
    /src/glsl/nir/nir_print.c: 299 in print_var_decl()
    293
    294           fprintf(fp, " (%s, %u)", loc, var->data.driver_location);
    295        }
    296
    297        fprintf(fp, "\n");
    298
    >>>     CID 1324642:  Null pointer dereferences  (REVERSE_INULL)
    >>>     Null-checking "state" suggests that it may be null, but it has already been dereferenced on all paths leading to the check.
    299        if (state) {
    300           _mesa_set_add(state->syms, name);
    301           _mesa_hash_table_insert(state->ht, var, name);
    302        }
    303     }
    304

Signed-off-by: Rob Clark <robclark@freedesktop.org>