Add Changelog entry

Signed-off-by: Paul Elliott <paul.elliott@arm.com>
diff --git a/ChangeLog.d/fix_ubsan_mp_aead_gcm.txt b/ChangeLog.d/fix_ubsan_mp_aead_gcm.txt
new file mode 100644
index 0000000..e4726a4
--- /dev/null
+++ b/ChangeLog.d/fix_ubsan_mp_aead_gcm.txt
@@ -0,0 +1,3 @@
+Bugfix
+   * Fix undefined behaviour (incrementing a NULL pointer by zero length) when
+     passing in zero length additional data to multipart AEAD.