output allow LWS_PRE+4 on top of rx_buffer_size for max send chunk

https://github.com/warmcat/libwebsockets/issues/522



Signed-off-by: Andy Green <andy@warmcat.com>

lib/output.c

@@ -127,7 +127,7 @@ int lws_issue_raw(struct lws *wsi, unsigned char *buf, size_t len)
 		lwsl_warn("** error invalid sock but expected to send\n");
 
 	/* limit sending */
-	n = wsi->protocol->rx_buffer_size;
+	n = wsi->protocol->rx_buffer_size + LWS_PRE + 4;
 	if (!n)
 		n = LWS_MAX_SOCKET_IO_BUF;
 	if (n > len)