Add changelog entry for #3998 fix

The fix was in https://github.com/ARMmbed/mbedtls/pull/4989.
We forgot to add a changelog entry.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
diff --git a/ChangeLog.d/dtls-cid-null.txt b/ChangeLog.d/dtls-cid-null.txt
new file mode 100644
index 0000000..b020616
--- /dev/null
+++ b/ChangeLog.d/dtls-cid-null.txt
@@ -0,0 +1,5 @@
+Bugfix
+   * In configurations with MBEDTLS_SSL_DTLS_CONNECTION_ID enabled but not
+     MBEDTLS_DEBUG_C, DTLS handshakes using CID would crash due to a null
+     pointer dereference. Fix this. Fixes #3998.
+     The fix was released, but not announced, in Mbed TLS 3.1.0.