bulk-checkin: zero-initialize hashfile_checkpoint

We declare a "struct hashfile_checkpoint" but only sometimes actually call hashfile_checkpoint() on it. That makes it not immediately obvious that it's valid when we later access its members. In fact, the code is fine: we fill it in unconditionally in the while(1) loop as long as "idx" is non-NULL. And then if "idx" is NULL, we exit early from the function (because we're just computing the hash, not actually writing), before we look at the struct. However, this does seem to confuse gcc 9.2.1's -Wmaybe-uninitialized when compiled with "-flto -O2" (probably because with LTO it can now realize that our call to hashfile_truncate() does not set the members either). Let's zero-initialize the struct to tell the compiler, as well as any readers of the code, that all is well. Reported-by: Stephan Beyer <s-beyer@gmx.net> Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>