radv: dont call calloc when BVH is empty

Usage of pointer returned by calloc(0) is UB.

Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/13503>

src/amd/vulkan/radv_acceleration_structure.c

@@ -473,6 +473,9 @@ bvh_opt_compare(const void *_a, const void *_b)
 static void
 optimize_bvh(const char *base_ptr, uint32_t *node_ids, uint32_t node_count)
 {
+   if (node_count == 0)
+      return;
+
    float bounds[6];
    for (unsigned i = 0; i < 3; ++i)
       bounds[i] = INFINITY;