Remove obsolete checks and limit horizontal factor. Fix #1076

src/ImageSharp/Formats/Jpeg/Components/Decoder/JpegComponent.cs

@@ -1,4 +1,4 @@
-// Copyright (c) Six Labors and contributors.
+// Copyright (c) Six Labors and contributors.
 // Licensed under the Apache License, Version 2.0.
 
 using System;
@@ -22,11 +22,8 @@ namespace SixLabors.ImageSharp.Formats.Jpeg.Components.Decoder
             this.Frame = frame;
             this.Id = id;
 
-            // Valid sampling factors are 1..2
-            if (horizontalFactor == 0
-                || verticalFactor == 0
-                || horizontalFactor > 2
-                || verticalFactor > 2)
+            // Validate sampling factors.
+            if (horizontalFactor == 0 || verticalFactor == 0)
             {
                 JpegThrowHelper.ThrowBadSampling();
             }
@@ -138,4 +135,4 @@ namespace SixLabors.ImageSharp.Formats.Jpeg.Components.Decoder
             this.SpectralBlocks = this.memoryAllocator.Allocate2D<Block8x8>(width, height, AllocationOptions.Clean);
         }
     }
-}
+}

src/ImageSharp/Formats/Jpeg/JpegDecoderCore.cs

@@ -764,7 +764,7 @@ namespace SixLabors.ImageSharp.Formats.Jpeg
                 for (int i = 0; i < this.ComponentCount; i++)
                 {
                     byte hv = this.temp[index + 1];
-                    int h = hv >> 4;
+                    int h = (hv >> 4) & 15;
                     int v = hv & 15;
 
                     if (maxH < h)