Fix resource leak when using mutex and ssl_cookie

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 c0039f4..52cf6d6 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -59,6 +59,8 @@
    * 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 2.4.1 branch released 2016-12-13
 
diff --git a/library/ssl_cookie.c b/library/ssl_cookie.c
index 9fb32de..caf1199 100644
--- a/library/ssl_cookie.c
+++ b/library/ssl_cookie.c
@@ -98,7 +98,7 @@
     mbedtls_md_free( &ctx->hmac_ctx );
 
 #if defined(MBEDTLS_THREADING_C)
-    mbedtls_mutex_init( &ctx->mutex );
+    mbedtls_mutex_free( &ctx->mutex );
 #endif
 
     mbedtls_zeroize( ctx, sizeof( mbedtls_ssl_cookie_ctx ) );