Fixed invalid baseline jpeg decoding

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

@@ -185,7 +185,7 @@ namespace SixLabors.ImageSharp.Formats.Jpeg.Components.Decoder
                         for (int y = 0; y < v; y++)
                         {
                             int blockRow = (mcuRow * v) + y;
-                            Span<Block8x8> blockSpan = component.SpectralBlocks.GetRowSpan(blockRow);
+                            Span<Block8x8> blockSpan = component.SpectralBlocks.GetRowSpan(y);
                             ref Block8x8 blockRef = ref MemoryMarshal.GetReference(blockSpan);
 
                             for (int x = 0; x < h; x++)
@@ -213,7 +213,8 @@ namespace SixLabors.ImageSharp.Formats.Jpeg.Components.Decoder
                 }
 
                 // convert from spectral to actual pixels via given converter
-                this.spectralConverter.ConvertStride(j, j);
+                this.spectralConverter.ConvertStride(j, 0);
+                this.spectralConverter.ClearStride(0);
             }
         }
 

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

@@ -108,6 +108,22 @@ namespace SixLabors.ImageSharp.Formats.Jpeg.Components.Decoder
             }
         }
 
+        public void ClearSpectralStride(int step)
+        {
+            int yBlockStart = step * this.BlockRowsPerStep;
+            for (int y = 0; y < this.BlockRowsPerStep; y++)
+            {
+                int yBlock = yBlockStart + y;
+
+                if (yBlock >= this.SizeInBlocks.Height)
+                {
+                    break;
+                }
+
+                this.Component.SpectralBlocks.GetRowSpan(yBlock).Clear();
+            }
+        }
+
         public void CopyBlocksToColorBuffer()
         {
             this.CopyBlocksToColorBuffer(this.currentComponentRowInBlocks);

src/ImageSharp/Formats/Jpeg/Components/Decoder/SpectralConverter{TPixel}.cs

@@ -21,6 +21,8 @@ namespace SixLabors.ImageSharp.Formats.Jpeg.Components.Decoder
 
         public abstract void ConvertStride(int step, int spectralStep);
 
+        public abstract void ClearStride(int spectralStep);
+
         public abstract void Dispose();
     }
 
@@ -134,6 +136,14 @@ namespace SixLabors.ImageSharp.Formats.Jpeg.Components.Decoder
             }
         }
 
+        public override void ClearStride(int spectralStep)
+        {
+            foreach (JpegComponentPostProcessor cpp in this.componentProcessors)
+            {
+                cpp.ClearSpectralStride(spectralStep);
+            }
+        }
+
         public override void Dispose()
         {
             foreach (JpegComponentPostProcessor cpp in this.componentProcessors)