libagx: add missing null pointer check

fixes KHR-GL46.pipeline_statistics_query_tests_ARB.functional_tess_queries
with hard fault

Signed-off-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/33682>

src/asahi/libagx/tessellation.cl

@@ -192,7 +192,9 @@ libagx_tess_setup_indirect(
    *vertex_output_buffer_ptr = (uintptr_t)(blob + vb_offs);
    p->counts = (global uint32_t *)(blob + count_offs);
 
-   ia->verts_per_instance = count;
+   if (ia) {
+      ia->verts_per_instance = count;
+   }
 
    /* If indexing is enabled, the third word is the offset into the index buffer
     * in elements. Apply that offset now that we have it. For a hardware