improve and bugfix websocket module

1). websocket.js should require tls module only in was a scheme. There are already runtime checks in websocket.js, this commits just catch the require statement. (#1891)
2). convert a signed char to unsigned short may cause websocket payload length overflow. like -1 for length 255 become 65535.

IoT.js-DCO-1.0-Signed-off-by: YDJiang jan4984@gmail.com

Author: jan4984

src/js/websocket.js

@@ -16,7 +16,12 @@
  var net = require('net');
  var util = require('util');
  var EventEmitter = require('events').EventEmitter;
- var tls = require('tls');
+ var tls;
+ try {
+   tls = require('tls');
+ } catch (e) {
+   tls = {};
+ }
 
 util.inherits(Websocket, EventEmitter);
 util.inherits(WebsocketClient, EventEmitter);

src/modules/iotjs_module_websocket.c

@@ -601,7 +601,8 @@ JS_FUNCTION(ws_receive) {
       if (current_buffer + sizeof(uint16_t) > current_buffer_end) {
         break;
       }
-      payload_len = (uint16_t)(current_buffer[0] << 8 | current_buffer[1]);
+      unsigned char *len_buffer = (unsigned char *)current_buffer;
+      payload_len = (uint16_t)(len_buffer[0] << 8 | len_buffer[1]);
       current_buffer += sizeof(uint16_t);
     } else if (!(payload_byte ^ WS_THREE_BYTES_LENGTH)) {
       uint64_t payload_64bit_len;