[TODO] Add item for fixing buffer error handling

TODO

diff --git a/TODO b/TODO
index bbb9797..eae8951 100644
--- a/TODO
+++ b/TODO
@@ -1,6 +1,7 @@
 - cmap14 support in get_glyph callback
 - Use size_t in sanitize?
 - Move unicode_funcs to buffer
+- Buffer error handling?
 
 hb-ot:
 - Fix ot query APIs