x509.c: Fix potential memory leak in X.509 self test

Found and fixed by Junhwan Park in #2106.

Signed-off-by: Junhwan Park <semoking@naver.com>
diff --git a/ChangeLog b/ChangeLog
index 66ecf97..e450e7b 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -251,6 +251,8 @@
      replacements of standard calloc/free functions through the macros
      MBEDTLS_PLATFORM_CALLOC_MACRO and MBEDTLS_PLATFORM_FREE_MACRO.
      Reported by ole-de and ddhome2006. Fixes #882, #1642 and #1706.
+   * Fix potential memory leak in X.509 self test. Found and fixed by
+     Junhwan Park, #2106.
 
 Changes
    * Removed support for Yotta as a build tool.