libpayload: don't test for unsigned >= 0

Change-Id: Ibeaf6de9505bc6f1e7358a4cfc80228dff7ddb69
Signed-off-by: Patrick Georgi <patrick@georgi-clan.de>
Found-by: Coverity Scan
Reviewed-on: http://review.coreboot.org/7978
Tested-by: build bot (Jenkins)
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>
1 file changed