Additional check in image_get_info

I noticed that if image_get_info is called in a situation where a folder is specified but there is no file, errors are returned (obviously).

I propose adding a check to make sure the $file variable is not a directory, with the following code change:

before

 function image_get_info($file) {
  if (!file_exists($file)) {
    return false;
  }

after

 function image_get_info($file) {
  if (!file_exists($file)||is_dir($file)) {
    return false;
  }