Re-add segmentation fault issues to todo list

I removed the segfault  note in the todo list when moving to libnitrokey
v3.3.  Yet the commit fixing this issue -- 7a8550d -- is not part of
that version.

1 files changed, 2 insertions, 0 deletions

diff --git a/TODO.md b/TODO.md
index 57ccf10..132086c 100644
--- a/TODO.md
+++ b/TODO.md
@@ -45,3 +45,5 @@
 - Improve log level documentation.
 - Clear passwords from memory.
 - Find a nicer syntax for the `write_config` test.
+- Fix segmentation faults when freeing string literals with old Nitrokey
+  versions (fixed in libnitrokey commit 7a8550d).