CVE-2020-27760

In `GammaImage()` of /MagickCore/enhance.c, depending on the `gamma` value, it's possible to trigger a divide-by-zero condition when a crafted input file is processed by ImageMagick. This could lead to an impact to application availability. The patch uses the `PerceptibleReciprocal()` to prevent the divide-by-zero from occurring. This flaw affects ImageMagick versions prior to ImageMagick 7.0.8-68.