"Test" command crash if the stream is stopped (i.e. when mp3client == NULL)

[Derek-K]

Esp-radio/Esp_radio.ino

Lines 2959 to 2963 in 56e57fc

	else if ( argument == "test" ) // Test command
	{
	sprintf ( reply, "Free memory is %d, ringbuf %d, stream %d",
	system_get_free_heap_size(), rcount, mp3client->available() ) ;
	}

"Test" command will crash if the stream is stopped.

Fix: check if mp3client is Null

  else if ( argument == "test" )                      // Test command
  {
    if (mp3client)
    {
      sprintf ( reply, "Free memory is %d, ringbuf %d, stream %d",
              system_get_free_heap_size(), rcount, mp3client->available() ) ;
    } else {
      sprintf ( reply, "Free memory is %d, ringbuf %d, No stream available",
              system_get_free_heap_size(), rcount ) ;      
    }
  }