Websockets - Avoids trying to use an empty buffer

  Sometimes, in a SSL secure connection, the receiveCallback is called with
an empty buffer and len == 0. At those times, the websocket client gets
confused.

  At connection time, it may not receive the Upgrade answer and disconnect.
At all other times, it would try to accumulate the payload until it exaust
all memory and fails.

app/websocket/websocketclient.c

@@ -257,6 +257,10 @@ static void ws_receiveCallback(void *arg, char *buf, unsigned short len) {
   os_timer_disarm(&ws->timeoutTimer); // reset ping check
   os_timer_arm(&ws->timeoutTimer, WS_PING_INTERVAL_MS, true);
 
+  if(len == 0){
+    NODE_DBG("len == 0");
+    return;
+  }
 
   char *b = buf;
   if (ws->frameBuffer != NULL) { // Append previous frameBuffer with new content
@@ -497,6 +501,10 @@ static void ws_initReceiveCallback(void *arg, char *buf, unsigned short len) {
   struct espconn *conn = (struct espconn *) arg;
   ws_info *ws = (ws_info *) conn->reverse;
 
+  if (len == 0){
+    NODE_DBG("len==0\n");
+    return;
+  }
   // Check server is switch protocols
   if (strstr(buf, WS_HTTP_SWITCH_PROTOCOL_HEADER) == NULL) {
       NODE_DBG("Server is not switching protocols\n");