notes: don't access hash of NULL object_id pointer

Check if note is NULL, as we already do for different purposes a few
lines above, and pass a NULL pointer to prepare_note_data() in that
case instead of trying to access the hash member.

Found with Clang's UBSan.

Signed-off-by: Rene Scharfe <l.s.r@web.de>
Acked-by: Brandon Williams <bmwill@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/notes.c

@@ -453,7 +453,7 @@ static int add(int argc, const char **argv, const char *prefix)
 			oid_to_hex(&object));
 	}
 
-	prepare_note_data(&object, &d, note->hash);
+	prepare_note_data(&object, &d, note ? note->hash : NULL);
 	if (d.buf.len || allow_empty) {
 		write_note_data(&d, new_note.hash);
 		if (add_note(t, &object, &new_note, combine_notes_overwrite))