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 8d0374f..31eb203 100644
--- a/library/ssl_cli.c
+++ b/library/ssl_cli.c
@@ -1304,6 +1304,8 @@
             }
 
             MBEDTLS_SSL_DEBUG_MSG( 1, ( "non-handshake message during renego" ) );
+
+            ssl->keep_current_message = 1;
             return( MBEDTLS_ERR_SSL_WAITING_SERVER_HELLO_RENEGO );
         }
 #endif /* MBEDTLS_SSL_RENEGOTIATION */