git-svn-id: https://www.imagemagick.org/subversion/ImageMagick/trunk@17097 aa41f4f7-0bf4-0310-aa73-e5a19afd5a74
Don't destroy a NULL "previous" image in the PNG decoder.


git-svn-id: https://www.imagemagick.org/subversion/ImageMagick/trunk@17099 aa41f4f7-0bf4-0310-aa73-e5a19afd5a74