Add tests for missing palette chunk

tests/ImageSharp.Tests/Formats/Png/PngDecoderTests.cs

@@ -265,6 +265,22 @@ namespace SixLabors.ImageSharp.Tests.Formats.Png
             Assert.Contains("PNG Image does not contain a data chunk", ex.Message);
         }
 
+        [Theory]
+        [WithFile(TestImages.Png.Bad.MissingPaletteChunk1, PixelTypes.Rgba32)]
+        [WithFile(TestImages.Png.Bad.MissingPaletteChunk2, PixelTypes.Rgba32)]
+        public void Decode_MissingPaletteChunk_ThrowsException<TPixel>(TestImageProvider<TPixel> provider)
+            where TPixel : unmanaged, IPixel<TPixel>
+        {
+            Exception ex = Record.Exception(
+                () =>
+                {
+                    using Image<TPixel> image = provider.GetImage(PngDecoder);
+                    image.DebugSave(provider);
+                });
+            Assert.NotNull(ex);
+            Assert.Contains("PNG Image does not contain palette chunk", ex.Message);
+        }
+
         [Theory]
         [WithFile(TestImages.Png.Bad.BitDepthZero, PixelTypes.Rgba32)]
         [WithFile(TestImages.Png.Bad.BitDepthThree, PixelTypes.Rgba32)]

tests/ImageSharp.Tests/TestImages.cs

@@ -127,6 +127,8 @@ namespace SixLabors.ImageSharp.Tests
                 public const string MissingDataChunk = "Png/xdtn0g01.png";
                 public const string WrongCrcDataChunk = "Png/xcsn0g01.png";
                 public const string CorruptedChunk = "Png/big-corrupted-chunk.png";
+                public const string MissingPaletteChunk1 = "Png/missing_plte.png";
+                public const string MissingPaletteChunk2 = "Png/missing_plte_2.png";
 
                 // Zlib errors.
                 public const string ZlibOverflow = "Png/zlib-overflow.png";

tests/Images/Input/Png/missing_plte.png

@@ -0,0 +1,3 @@
+version https://git-lfs.github.com/spec/v1
+oid sha256:73fd17a394f8258f4767986bc427c0160277819349c937f18cb29044e7549bc8
+size 506

tests/Images/Input/Png/missing_plte_2.png

@@ -0,0 +1,3 @@
+version https://git-lfs.github.com/spec/v1
+oid sha256:797844db61a937c6f31ecb392c8416fbf106017413ba55c6576e0b1fcfc1cf9c
+size 597