SECURITY UPDATE: stack buffer overflow in MNG/JNG encoder — missing NULL
check after ImageToBlob() in WriteOneJNGImage could propagate a NULL blob
pointer into later stack buffer operations (GHSA-7h7q-j33q-hvpf).
debian/patches/CVE-2026-28690.patch: bail out of WriteOneJNGImage when
ImageToBlob() returns NULL, destroying jpegimage and jpegimage_info
before returning MagickFalse (upstream e6e874875e48dd9838acca3bd22c14a4d2f1b3ca)