Fix memory leak upon ciphersuite mismatch during session resumption

Signed-off-by: Hanno Becker <hanno.becker@arm.com>
diff --git a/library/ssl_srv.c b/library/ssl_srv.c
index 196f69d..213cd3c 100644
--- a/library/ssl_srv.c
+++ b/library/ssl_srv.c
@@ -2810,7 +2810,7 @@
 
 exit:
 
-    mbedtls_platform_zeroize( &session_tmp, sizeof( session_tmp ) );
+    mbedtls_ssl_session_free( &session_tmp );
 }
 
 static int ssl_write_server_hello( mbedtls_ssl_context *ssl )