Backport 2.1:Resource leak in ssl_cookie and mutex

When using ssl_cookie with MBEDTLS_THREADING_C, fix a resource leak caused by
initiating a mutex in mbedtls_ssl_cookie_free instead of freeing it.
Raised and fix suggested by lan Gillingham in the mbed TLS forum
Tracked in #771
diff --git a/ChangeLog b/ChangeLog
index bb15ee2..32d76df 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -56,6 +56,8 @@
    * Fixed the templates used to generate project and solution files for Visual
      Studio 2015 as well as the files themselves, to remove a build warning
      generated in Visual Studio 2015. Reported by Steve Valliere. #742
+   * Fix a resource leak in ssl_cookie, when using MBEDTLS_THREADING_C.
+     Raised and fix suggested by Alan Gillingham in the mbed TLS forum. #771
 
 = mbed TLS 2.1.6 branch released 2016-10-17