Fix mbedtls_ssl_read

Don't fetch a new record in mbedtls_ssl_read_record_layer as long as
an application data record is being processed.
diff --git a/library/ssl_cli.c b/library/ssl_cli.c
index c12543f..5f5beec 100644
--- a/library/ssl_cli.c
+++ b/library/ssl_cli.c
@@ -1195,6 +1195,8 @@
             }
 
             SSL_DEBUG_MSG( 1, ( "non-handshake message during renego" ) );
+
+            ssl->keep_current_message = 1;
             return( POLARSSL_ERR_SSL_WAITING_SERVER_HELLO_RENEGO );
         }
 #endif /* POLARSSL_SSL_RENEGOTIATION */