tools/bootconfig: Fix buf leaks in apply_xbc

If data calloc failed, free the buf before return.

Link: https://lore.kernel.org/all/20260520030126.147782-1-lihongtao@kylinos.cn/

Fixes: 950313ebf79c ("tools: bootconfig: Add bootconfig command")
Signed-off-by: Hongtao Lee <lihongtao@kylinos.cn>
Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>