Can't reliably store images/fonts in file_directory_path() .'/textimage/'

Bit of an edge case, but it did cause half an hour of hair-tearing. If you use file_directory_path() .'/textimage/' to store your images and fonts, they may be unselectable in the admin UI, because the list of available files is retrieved during a while loop that will halt when it finds a directory called '0', the preview image cache directory.

The attached little patch will fix this. I don't think there are any other places in the code where this is a problem.