Prevent underflow

src/ImageSharp/Formats/Webp/AlphaDecoder.cs

@@ -6,6 +6,7 @@ using System.Diagnostics.CodeAnalysis;
 using System.Runtime.CompilerServices;
 using System.Runtime.InteropServices;
 using System.Runtime.Intrinsics;
+using System.Runtime.Intrinsics.Arm;
 using System.Runtime.Intrinsics.X86;
 using SixLabors.ImageSharp.Formats.Webp.BitReader;
 using SixLabors.ImageSharp.Formats.Webp.Lossless;
@@ -311,18 +312,15 @@ internal class AlphaDecoder : IDisposable
 
     private static void HorizontalUnfilter(Span<byte> prev, Span<byte> input, Span<byte> dst, int width)
     {
-        if (Sse2.IsSupported)
+        // TODO: Investigate AdvSim support for this method.
+        if (Sse2.IsSupported && width >= 9)
         {
             dst[0] = (byte)(input[0] + (prev.IsEmpty ? 0 : prev[0]));
-            if (width <= 1)
-            {
-                return;
-            }
-
             nuint i;
             Vector128<int> last = Vector128<int>.Zero.WithElement(0, dst[0]);
             ref byte srcRef = ref MemoryMarshal.GetReference(input);
             ref byte dstRef = ref MemoryMarshal.GetReference(dst);
+
             for (i = 1; i <= (uint)width - 8; i += 8)
             {
                 Vector128<long> a0 = Vector128.Create(Unsafe.As<byte, long>(ref Unsafe.Add(ref srcRef, i)), 0);

tests/ImageSharp.Tests/Formats/WebP/WebpDecoderTests.cs

@@ -439,6 +439,17 @@ public class WebpDecoderTests
         image.CompareToOriginal(provider, ReferenceDecoder);
     }
 
+    // https://github.com/SixLabors/ImageSharp/issues/2670
+    [Theory]
+    [WithFile(Lossy.Issue2670, PixelTypes.Rgba32)]
+    public void WebpDecoder_CanDecode_Issue2670<TPixel>(TestImageProvider<TPixel> provider)
+        where TPixel : unmanaged, IPixel<TPixel>
+    {
+        using Image<TPixel> image = provider.GetImage(WebpDecoder.Instance);
+        image.DebugSave(provider);
+        image.CompareToOriginal(provider, ReferenceDecoder);
+    }
+
     [Theory]
     [WithFile(Lossless.LossLessCorruptImage3, PixelTypes.Rgba32)]
     public void WebpDecoder_ThrowImageFormatException_OnInvalidImages<TPixel>(TestImageProvider<TPixel> provider)

tests/ImageSharp.Tests/TestImages.cs

@@ -804,6 +804,7 @@ public static class TestImages
             public const string Issue1594 = "Webp/issues/Issue1594.webp";
             public const string Issue2243 = "Webp/issues/Issue2243.webp";
             public const string Issue2257 = "Webp/issues/Issue2257.webp";
+            public const string Issue2670 = "Webp/issues/Issue2670.webp";
         }
     }
 

tests/Images/Input/Webp/issues/Issue2670.webp

@@ -0,0 +1,3 @@
+version https://git-lfs.github.com/spec/v1
+oid sha256:23ad5eb449f693af68e51dd108a6b9847a8eb48b82ca5b848395a54c2e0be08f
+size 152