Add additional checks

src/ImageSharp/Processing/Processors/Dithering/ErrorDither.cs

@@ -27,6 +27,8 @@ namespace SixLabors.ImageSharp.Processing.Processors.Dithering
         [MethodImpl(InliningOptions.ShortMethod)]
         public ErrorDither(in DenseMatrix<float> matrix, int offset)
         {
+            Guard.MustBeGreaterThan(offset, 0, nameof(offset));
+
             this.matrix = matrix;
             this.offset = offset;
         }

src/ImageSharp/Processing/Processors/Dithering/OrderedDither.cs

@@ -24,6 +24,8 @@ namespace SixLabors.ImageSharp.Processing.Processors.Dithering
         [MethodImpl(InliningOptions.ShortMethod)]
         public OrderedDither(uint length)
         {
+            Guard.MustBeGreaterThan(length, 0, nameof(length));
+
             DenseMatrix<uint> ditherMatrix = OrderedDitherFactory.CreateDitherMatrix(length);
 
             // Create a new matrix to run against, that pre-thresholds the values.