fsck: handle NULL return of lookup_blob() and lookup_tree()rs/fsck-null-return-from-lookup

lookup_blob() and lookup_tree() can return NULL if they find an object
of an unexpected type.  Accessing the object member is undefined in that
case.  Cast the result to a struct object pointer instead; we can do
that because object is the first member of all object types.  This trick
is already used in other places in the code.

An error message is already shown by object_as_type(), which is called
by the lookup functions.  The walk callback functions are expected to
handle NULL object pointers passed to them, but put_object_name() needs
a valid object, so avoid calling it without one.

Suggested-by: SZEDER Gábor <szeder.dev@gmail.com>
Helped-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Rene Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 4 insertions, 4 deletions

diff --git a/fsck.c b/fsck.c
index b4204d772b..b1579c7e28 100644
--- a/fsck.c
+++ b/fsck.c
@@ -358,15 +358,15 @@ static int fsck_walk_tree(struct tree *tree, void *data, struct fsck_options *op
 			continue;
 
 		if (S_ISDIR(entry.mode)) {
-			obj = &lookup_tree(entry.oid)->object;
-			if (name)
+			obj = (struct object *)lookup_tree(entry.oid);
+			if (name && obj)
 				put_object_name(options, obj, "%s%s/", name,
 					entry.path);
 			result = options->walk(obj, OBJ_TREE, data, options);
 		}
 		else if (S_ISREG(entry.mode) || S_ISLNK(entry.mode)) {
-			obj = &lookup_blob(entry.oid)->object;
-			if (name)
+			obj = (struct object *)lookup_blob(entry.oid);
+			if (name && obj)
 				put_object_name(options, obj, "%s%s", name,
 					entry.path);
 			result = options->walk(obj, OBJ_BLOB, data, options);