Throw exception when malformed apple png chunk is detected (#410)

src/ImageSharp/Formats/Png/PngDecoderCore.cs

@@ -9,7 +9,7 @@ using System.IO.Compression;
 using System.Runtime.CompilerServices;
 using System.Runtime.InteropServices;
 using System.Text;
-using SixLabors.ImageSharp.Advanced;
+
 using SixLabors.ImageSharp.Formats.Png.Chunks;
 using SixLabors.ImageSharp.Formats.Png.Filters;
 using SixLabors.ImageSharp.Formats.Png.Zlib;
@@ -215,6 +215,9 @@ namespace SixLabors.ImageSharp.Formats.Png
                             case PngChunkType.End:
                                 this.isEndChunkReached = true;
                                 break;
+                            case PngChunkType.MalformedApple:
+                                PngThrowHelper.ThrowInvalidChunkType("Malformed Apple PNG detected! This PNG file is not conform to the specification and cannot be decoded.");
+                                break;
                         }
                     }
                     finally
@@ -1039,7 +1042,7 @@ namespace SixLabors.ImageSharp.Formats.Png
 
                 var uncompressedBytes = new List<byte>();
 
-                // Note: this uses the a buffer which is only 4 bytes long to read the stream, maybe allocating a larger buffer makes sense here.
+                // Note: this uses a buffer which is only 4 bytes long to read the stream, maybe allocating a larger buffer makes sense here.
                 int bytesRead = inflateStream.CompressedStream.Read(this.buffer, 0, this.buffer.Length);
                 while (bytesRead != 0)
                 {