Let check_preimage() use memset() to initialize "struct checkout"

Let check_preimage() use memset() to initialize "struct checkout"

Every code site except check_preimage() uses either memset() or declares
a static instance of "struct checkout" to achieve proper initialization.
Lets use memset() instead of explicit initialization of all members here
too to be on the safe side in case this structure is expanded someday.

Signed-off-by: Jens Lehmann <Jens.Lehmann@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>