Handle corrupted data portions. Fix #358

src/ImageSharp/Formats/Png/PngDecoderCore.cs

@@ -1124,12 +1124,23 @@ namespace SixLabors.ImageSharp.Formats.Png
         {
             var chunk = new PngChunk();
             this.ReadChunkLength(chunk);
-            if (chunk.Length < 0)
+
+            if (chunk.Length == -1)
             {
+                // IEND
                 return null;
             }
 
+            if (chunk.Length < 0 || chunk.Length > this.currentStream.Length - this.currentStream.Position)
+            {
+                // Not a valid chunk so we skip back all but one of the four bytes we have just read.
+                // That lets us read one byte at a time until we reach a known chunk.
+                this.currentStream.Position -= 3;
+                return chunk;
+            }
+
             this.ReadChunkType(chunk);
+
             if (chunk.Type == PngChunkTypes.Data)
             {
                 return chunk;

tests/ImageSharp.Tests/TestImages.cs

@@ -20,6 +20,7 @@ namespace SixLabors.ImageSharp.Tests
             public const string Blur = "Png/blur.png";
             public const string Indexed = "Png/indexed.png";
             public const string Splash = "Png/splash.png";
+            public const string CorruptedChunk = "Png/big-corrupted-chunk.png";
             public const string Cross = "Png/cross.png";
             public const string Powerpoint = "Png/pp.png";
             public const string SplashInterlaced = "Png/splash-interlaced.png";