Added missing clear for the previousScanline when going to the next pass

src/ImageSharp/Formats/Png/PngDecoderCore.cs

@@ -515,7 +515,7 @@ namespace ImageSharp.Formats
                     this.currentRowBytesRead = 0;
 
                     Span<byte> scanSpan = this.scanline.Slice(0, bytesPerInterlaceScanline);
-                    Span<byte> prevSpan = this.previousScanline.Span.Slice(0, bytesPerInterlaceScanline);
+                    Span<byte> prevSpan = this.previousScanline.Slice(0, bytesPerInterlaceScanline);
                     var filterType = (FilterType)scanSpan[0];
 
                     switch (filterType)
@@ -556,6 +556,8 @@ namespace ImageSharp.Formats
                 }
 
                 this.pass++;
+                this.previousScanline.Clear();
+
                 if (this.pass < 7)
                 {
                     this.currentRow = Adam7FirstRow[this.pass];