tools/bootconfig: fix fd leak in load_xbc_file() on fstat failure

If fstat() fails after open() succeeds, the function returns without
closing the file descriptor. Also preserve errno across close(), since
close() may overwrite it before the error is returned.

Link: https://lore.kernel.org/all/20260318155847.78065-3-objecting@objecting.org/

Fixes: 950313ebf79c ("tools: bootconfig: Add bootconfig command")
Signed-off-by: Josh Law <objecting@objecting.org>
Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>