CVE-2020-25666

There are 4 places in HistogramCompare() in MagickCore/histogram.c where an integer overflow is possible during simple math calculations. This occurs in the rgb values and `count` value for a color. The patch uses casts to `ssize_t` type for these calculations, instead of `int`. This flaw could impact application reliability in the event that ImageMagick processes a crafted input file. This flaw affects ImageMagick versions prior to 7.0.9-0.