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

[Junio C Hamano <gitster@pobox.com>]

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>
---
 object-file.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/object-file.c b/object-file.c
index 1a24f08978..bd580ef032 100644
--- a/object-file.c
+++ b/object-file.c
@@ -720,7 +720,7 @@ struct odb_transaction_files {
 static void prepare_loose_object_transaction(struct odb_transaction *base)
 {
 	struct odb_transaction_files *transaction =
-		container_of(base, struct odb_transaction_files, base);
+		container_of_or_null(base, struct odb_transaction_files, base);
 
 	/*
 	 * We lazily create the temporary object directory
@@ -740,7 +740,7 @@ static void fsync_loose_object_transaction(struct odb_transaction *base,
 					   int fd, const char *filename)
 {
 	struct odb_transaction_files *transaction =
-		container_of(base, struct odb_transaction_files, base);
+		container_of_or_null(base, struct odb_transaction_files, base);
 
 	/*
 	 * If we have an active ODB transaction, we issue a call that
-- 
2.53.0-455-gd82541b467