Adapt ChangeLog

Add note about fix of memory leak in RSA self test.
diff --git a/ChangeLog b/ChangeLog
index 6e96fcd..027c560 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -9,6 +9,7 @@
 Bugfix
    * Fix ssl_parse_record_header() to silently discard invalid DTLS records
      as recommended in RFC 6347 Section 4.1.2.7.
+   * Fix memory leak in RSA self test.
 
 = mbed TLS 2.1.9 branch released 2017-08-10