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>

path-walk.c

@@ -171,7 +171,7 @@ static int add_tree_entries(struct path_walk_context *ctx,
 
 		if (!o) {
 			error(_("failed to find object %s"),
-			      oid_to_hex(&o->oid));
+			      oid_to_hex(&entry.oid));
 			return -1;
 		}