Throw exception, if palette chunk is missing

src/ImageSharp/Formats/Png/PngScanlineProcessor.cs

@@ -240,6 +240,11 @@ namespace SixLabors.ImageSharp.Formats.Png
             byte[] paletteAlpha)
             where TPixel : unmanaged, IPixel<TPixel>
         {
+            if (palette.IsEmpty)
+            {
+                PngThrowHelper.ThrowMissingPalette();
+            }
+
             TPixel pixel = default;
             ref byte scanlineSpanRef = ref MemoryMarshal.GetReference(scanlineSpan);
             ref TPixel rowSpanRef = ref MemoryMarshal.GetReference(rowSpan);

src/ImageSharp/Formats/Png/PngThrowHelper.cs

@@ -21,6 +21,9 @@ namespace SixLabors.ImageSharp.Formats.Png
         [MethodImpl(InliningOptions.ColdPath)]
         public static void ThrowNoData() => throw new InvalidImageContentException("PNG Image does not contain a data chunk");
 
+        [MethodImpl(InliningOptions.ColdPath)]
+        public static void ThrowMissingPalette() => throw new InvalidImageContentException("PNG Image does not contain palette chunk");
+
         [MethodImpl(InliningOptions.ColdPath)]
         public static void ThrowInvalidChunkType() => throw new InvalidImageContentException("Invalid PNG data.");