Update image.php

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`.
Merge pull request #2026 from j-ed/master

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