Debian: CVE-2020-27753: imagemagick -- security update

There are several memory leaks in the MIFF coder in /coders/miff.c due to improper image depth values, which can be triggered by a specially crafted input file. These leaks could potentially lead to an impact to application availability or cause a denial of service. It was originally reported that the issues were in `AcquireMagickMemory()` because that is where LeakSanitizer detected the leaks, but the patch resolves issues in the MIFF coder, which incorrectly handles data being passed to `AcquireMagickMemory()`. This flaw affects ImageMagick versions prior to 7.0.9-0.