fix missing sanity check if BT

lua_script_examples/echo-serial/echo-serial.lua

@@ -0,0 +1,26 @@
+--[[
+    Echo Serial
+
+    This example echo serial outout to serial with echo: header 
+    Put this script in the ESP3D flash file system and call it from the init script in 
+    configuration.h.
+    #define ESP_AUTOSTART_SCRIPT "[ESP300]/FS/detectpush.lua"
+
+    Be sure to have the following line comment out in the configure.h:
+    #define ESP_LUA_INTERPRETER_FEATURE
+    ]]--
+
+-- Setup
+local availableMsg = 0
+local message = ""
+while (true) do
+    -- check if have data
+    availableMsg = available()
+    -- if any message
+    if (availableMsg > 0) then
+        message = "echo:" .. readData()
+        print(message)
+    end
+    -- yield to other tasks
+    yield()
+end