Don't handle "previous" image in the JNG reader. JNG is a single-image format.


git-svn-id: https://www.imagemagick.org/subversion/ImageMagick/trunk@17079 aa41f4f7-0bf4-0310-aa73-e5a19afd5a74
git-svn-id: https://www.imagemagick.org/subversion/ImageMagick/trunk@17082 aa41f4f7-0bf4-0310-aa73-e5a19afd5a74