Backport #3105 to v3 branch

Author: JimBobSquarePants

src/ImageSharp/Common/Helpers/SimdUtils.HwIntrinsics.cs

@@ -1077,8 +1077,10 @@ internal static void UnpackToRgbPlanesAvx2Reduce(
             Vector256<byte> rgb, rg, bx;
             Vector256<float> r, g, b;
 
+            // Each iteration consumes 8 Rgb24 pixels (24 bytes) but starts with a 32-byte load,
+            // so we need 3 extra pixels of addressable slack beyond the vectorized chunk.
             const int bytesPerRgbStride = 24;
-            nuint count = (uint)source.Length / 8;
+            nuint count = source.Length > 3 ? (uint)(source.Length - 3) / 8 : 0;
             for (nuint i = 0; i < count; i++)
             {
                 rgb = Avx2.PermuteVar8x32(Unsafe.AddByteOffset(ref rgbByteSpan, (uint)(bytesPerRgbStride * i)).AsUInt32(), extractToLanesMask).AsByte();
@@ -1098,10 +1100,10 @@ internal static void UnpackToRgbPlanesAvx2Reduce(
             }
 
             int sliceCount = (int)(count * 8);
-            redChannel = redChannel.Slice(sliceCount);
-            greenChannel = greenChannel.Slice(sliceCount);
-            blueChannel = blueChannel.Slice(sliceCount);
-            source = source.Slice(sliceCount);
+            redChannel = redChannel[sliceCount..];
+            greenChannel = greenChannel[sliceCount..];
+            blueChannel = blueChannel[sliceCount..];
+            source = source[sliceCount..];
         }
     }
 }

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

@@ -113,9 +113,9 @@ private void ConvertStride(int spectralStep)
             Span<TPixel> sourceRow = this.pixelBuffer.DangerousGetRowSpan(srcIndex);
             PixelOperations<TPixel>.Instance.UnpackIntoRgbPlanes(rLane, gLane, bLane, sourceRow);
 
-            rLane.Slice(paddingStartIndex).Fill(rLane[paddingStartIndex - 1]);
-            gLane.Slice(paddingStartIndex).Fill(gLane[paddingStartIndex - 1]);
-            bLane.Slice(paddingStartIndex).Fill(bLane[paddingStartIndex - 1]);
+            rLane.Slice(paddingStartIndex, paddedPixelsCount).Fill(rLane[paddingStartIndex - 1]);
+            gLane.Slice(paddingStartIndex, paddedPixelsCount).Fill(gLane[paddingStartIndex - 1]);
+            bLane.Slice(paddingStartIndex, paddedPixelsCount).Fill(bLane[paddingStartIndex - 1]);
 
             // Convert from rgb24 to target pixel type
             var values = new JpegColorConverterBase.ComponentValues(this.componentProcessors, y);