message: PipeWire capturer: check existence of cursor metadata
  Check whether there are any cursor metadata before we try to validate
  and use them, otherwise we might crash on this.
  Cr-Commit-Position: refs/heads/main@{#36240}
diff: @@ -650,7 +650,7 @@ void SharedScreenCastStreamPrivate::ProcessBuffer(pw_buffer* buffer) {
  const struct spa_meta_cursor* cursor =
  static_cast<struct spa_meta_cursor*>(spa_buffer_find_meta_data(
  spa_buffer, SPA_META_Cursor, sizeof(*cursor)));
  - if (spa_meta_cursor_is_valid(cursor)) {
  + if (cursor && spa_meta_cursor_is_valid(cursor)) {
  struct spa_meta_bitmap* bitmap = nullptr;
  if (cursor->bitmap_offset)
  