Fix zero level mipmaps in KTX2

Author: Erik-White

src/ImageSharp.Textures/Formats/Ktx2/Ktx2DecoderCore.cs

@@ -69,7 +69,8 @@ public Texture DecodeTexture(Stream stream)
             int width = (int)this.ktxHeader.PixelWidth;
             int height = (int)this.ktxHeader.PixelHeight;
 
-            var levelIndices = new LevelIndex[this.ktxHeader.LevelCount];
+            // Zero levels means only the base level is present, so we must read at least 1.
+            LevelIndex[] levelIndices = new LevelIndex[Math.Max(1, this.ktxHeader.LevelCount)];
             for (int i = 0; i < levelIndices.Length; i++)
             {
                 stream.Read(this.buffer, 0, 24);

tests/ImageSharp.Textures.Tests/Formats/Ktx2/Ktx2DecoderTests.cs

@@ -0,0 +1,34 @@
+// Copyright (c) Six Labors.
+// Licensed under the Six Labors Split License.
+
+using System.ComponentModel;
+using SixLabors.ImageSharp.Textures.Formats.Ktx2;
+using SixLabors.ImageSharp.Textures.TextureFormats;
+
+namespace SixLabors.ImageSharp.Textures.Tests.Formats.Ktx2;
+
+[Trait("Format", "Ktx2")]
+public class Ktx2DecoderTests
+{
+    private static readonly Ktx2Decoder Ktx2Decoder = new();
+
+    [Fact]
+    [Description("Ensure that a single mipmap level does not result in an empty mipmap collection")]
+    public void Ktx2Decoder_LevelCountZero_DecodesBaseLevelMipMap()
+    {
+        string path = Path.Combine(
+            TestEnvironment.InputImagesDirectoryFullPath,
+            "Ktx2",
+            "Flat",
+            "rgba32-srgb-mips.ktx2");
+
+        using FileStream fileStream = File.OpenRead(path);
+        using Texture texture = Ktx2Decoder.DecodeTexture(Configuration.Default, fileStream);
+
+        FlatTexture flatTexture = texture as FlatTexture;
+        Assert.NotNull(flatTexture);
+        Assert.Single(flatTexture.MipMaps);
+        Assert.Equal(256, flatTexture.MipMaps[0].GetImage().Width);
+        Assert.Equal(256, flatTexture.MipMaps[0].GetImage().Height);
+    }
+}

tests/Images/Input/Ktx2/Flat/rgba32-srgb-mips.ktx2

@@ -0,0 +1,3 @@
+version https://git-lfs.github.com/spec/v1
+oid sha256:90846eff319fdfd03121b3e0300fc4106ddcc2a4df881598cca9978e04710004
+size 393468