Input validation

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.