nir: only dereference undef after NULL check. (v2)

Pointed out by coverity.

v2: nuke line, Jason pointed out the constructor does it.
Signed-off-by: Dave Airlie <airlied@redhat.com>

src/compiler/nir/nir_builder.h

@@ -79,7 +79,6 @@ nir_ssa_undef(nir_builder *build, unsigned num_components, unsigned bit_size)
 {
    nir_ssa_undef_instr *undef =
       nir_ssa_undef_instr_create(build->shader, num_components, bit_size);
-   undef->def.bit_size = bit_size;
    if (!undef)
       return NULL;