nir/lower_clip_cull: Fix an incorrect assert

Copy+paste error.  It was supposed to test cull and not clip.

Fixes: 4e69fba534 "nir: Rewrite lower_clip_cull_distance_arrays..."
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=109717
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>

src/compiler/nir/nir_lower_clip_cull_distance_arrays.c

@@ -101,7 +101,7 @@ combine_clip_cull(nir_shader *nir,
    }
 
    if (cull) {
-      assert(clip->data.compact);
+      assert(cull->data.compact);
       cull->data.how_declared = nir_var_hidden;
       cull->data.location = VARYING_SLOT_CLIP_DIST0 + clip_array_size / 4;
       cull->data.location_frac = clip_array_size % 4;