tools: kwbimage: Fix check for v0 extended header checksum

tools: kwbimage: Fix check for v0 extended header checksum

Extended header checksum for v0 image is present only in the case when
extended header is present. Skip checksum validation if extended header
is not present.

Signed-off-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Marek Behún <marek.behun@nic.cz>
Reviewed-by: Stefan Roese <sr@denx.de>
Reviewed-by: Chris Packham <judge.packham@gmail.com>
Tested-by: Chris Packham <judge.packham@gmail.com>