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>