Fix concurrent png decoding

src/ImageSharp.Formats.Png/PngDecoderCore.cs

@@ -361,7 +361,8 @@ namespace ImageSharp.Formats
             byte[] previousScanline = ArrayPool<byte>.Shared.Rent(this.bytesPerScanline);
             byte[] scanline = ArrayPool<byte>.Shared.Rent(this.bytesPerScanline);
 
-            // Zero out the previousScanline, because the bytes that are rented from the arraypool may not be zero.
+            // Zero out the scanlines, because the bytes that are rented from the arraypool may not be zero.
+            Array.Clear(scanline, 0, this.bytesPerScanline);
             Array.Clear(previousScanline, 0, this.bytesPerScanline);
 
             try
@@ -437,7 +438,8 @@ namespace ImageSharp.Formats
             {
                 for (int pass = 0; pass < 7; pass++)
                 {
-                    // Zero out the previousScanline, because the bytes that are rented from the arraypool may not be zero.
+                    // Zero out the scanlines, because the bytes that are rented from the arraypool may not be zero.
+                    Array.Clear(scanline, 0, this.bytesPerScanline);
                     Array.Clear(previousScanline, 0, this.bytesPerScanline);
 
                     int y = Adam7FirstRow[pass];