ref: initialize "fsck_ref_report" with zero

ref: initialize "fsck_ref_report" with zero

In "fsck.c::fsck_refs_error_function", we need to tell whether "oid" and
"referent" is NULL. So, we need to always initialize these parameters to
NULL instead of letting them point to anywhere when creating a new
"fsck_ref_report" structure.

The original code explicitly initializes the "path" member in the
"struct fsck_ref_report" to NULL (which implicitly 0-initializes other
members in the struct). It is more customary to use "{ 0 }" to express
that we are 0-initializing everything. In order to align with the
codebase, initialize "fsck_ref_report" with zero.

Mentored-by: Patrick Steinhardt <ps@pks.im>
Mentored-by: Karthik Nayak <karthik.188@gmail.com>
Signed-off-by: shejialuo <shejialuo@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>