object-file.c: avoid container_of() of a NULL container

object-file.c: avoid container_of() of a NULL container

Even though the "struct odb_transaction" member is at the beginning
of the containing "struct odb_transaction_files", i.e., at offset 0,
using container_of() to add offset 0 to a NULL pointer gets flagged
as a bad behaviour under SANITIZE=undefined.

Use container_of_or_null() to work around this issue.

Helped-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>