path-walk: fix NULL pointer dereference in error message

When lookup_tree() or lookup_blob() cannot find a tree entry's object, 'o' is set to NULL via: o = child ? &child->object : NULL; The subsequent null-check catches this correctly, but then dereferences 'o' to format the error message: error(_("failed to find object %s"), oid_to_hex(&o->oid)); This causes a segfault instead of the intended diagnostic output. Fix this by using &entry.oid instead. 'entry' is the struct name_entry populated by tree_entry() on each loop iteration and holds the OID of the failing lookup -- which is exactly what the error should report. This crash is reachable via git-backfill(1) when a tree entry's object is absent from the local object database. Signed-off-by: Yuvraj Singh Chauhan <ysinghcin@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>