function handleCallback(err, data) {
      if (!ended) {
        ended = true;
        callback(err, data);
      }
    }
Run ffprobe on last specified input

@method FfmpegCommand#ffprobe
@category Metadata

@param {?Number} [index] 0-based index of input to probe (defaults to last input)
@param {?String[]} [options] array of output options to return
@param {FfmpegCommand~ffprobeCallback} callback callback function