tools: kwbimage: Check the return value of image_headersz_v1()

Function image_headersz_v1() may return zero on fatal errors.
In this case the function already printed an error message.

Check the return value of image_headersz_v1() in kwbimage_generate(),
and exit on zero value with EXIT_FAILURE.

Signed-off-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Marek Behún <marek.behun@nic.cz>
Reviewed-by: Stefan Roese <sr@denx.de>