nir/lower_point_size: skip non-var derefs

these can happen depending on pass order, otherwise we crash on the null
pointer.

Signed-off-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/32564>

src/compiler/nir/nir_lower_point_size.c

@@ -43,6 +43,9 @@ lower_point_size_intrin(nir_builder *b, nir_intrinsic_instr *intr, void *data)
    if (intr->intrinsic == nir_intrinsic_store_deref) {
       nir_deref_instr *deref = nir_src_as_deref(intr->src[0]);
       nir_variable *var = nir_deref_instr_get_variable(deref);
+      if (!var)
+         return false;
+
       location = var->data.location;
       psiz_src = &intr->src[1];
    } else if (intr->intrinsic == nir_intrinsic_store_output ||