do not try to extract image infos from 0- or better less than 12-byte length files.

don't try to extract image infos from 0- or better less than 12 byte length files. See description of `exif_imagetype` function at `http://php.net/manual/de/function.exif-imagetype.php`.

This should fix a problem describe in https://github.com/owncloud/apps/issues/646