Fix certificate management when freeing handshake 

Signed-off-by: Andrzej Kurek <andrzej.kurek@arm.com>
diff --git a/library/ssl_tls.c b/library/ssl_tls.c
index f20faf9..f7f9e32 100644
--- a/library/ssl_tls.c
+++ b/library/ssl_tls.c
@@ -12099,12 +12099,11 @@
     defined(MBEDTLS_SSL_KEEP_PEER_CERTIFICATE)
     if( ssl->session_negotiate )
     {
-        mbedtls_x509_crt_free( ssl->session_negotiate->peer_cert );
+        ssl_clear_peer_cert( ssl->session_negotiate );
     }
     if( ssl->session )
     {
-        mbedtls_free( ssl->session->peer_cert );
-        ssl->session->peer_cert = NULL;
+        ssl_clear_peer_cert( ssl->session );
     }
 #endif /* MBEDTLS_SSL_FREE_SERVER_CERTIFICATE */