x86: zimage: Sanity-check the kernel version before printing it

With Chrome OS the kernel setup block is stored in a separate place from
the kernel, so it is not possible to access the kernel version string.
At present, garbage is printed.

Add a sanity check to avoid this.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>