Apply suggestions from code review

Co-authored-by: Günther Foidl <gue@korporal.at>

src/ImageSharp/Formats/Png/PngDecoderCore.cs

@@ -1186,7 +1186,7 @@ namespace SixLabors.ImageSharp.Formats.Png
         private void ReadColorProfileChunk(ImageMetadata metadata, ReadOnlySpan<byte> data)
         {
             int zeroIndex = data.IndexOf((byte)0);
-            if (zeroIndex < PngConstants.MinTextKeywordLength || zeroIndex > PngConstants.MaxTextKeywordLength)
+            if (zeroIndex is < PngConstants.MinTextKeywordLength or > PngConstants.MaxTextKeywordLength)
             {
                 return;
             }

src/ImageSharp/Formats/Png/PngEncoderCore.cs

@@ -694,11 +694,13 @@ namespace SixLabors.ImageSharp.Formats.Png
                 int bytesWritten = PngConstants.XmpKeyword.Length;
 
                 // Write the iTxt header (all zeros in this case).
-                payload[bytesWritten++] = 0;
-                payload[bytesWritten++] = 0;
-                payload[bytesWritten++] = 0;
-                payload[bytesWritten++] = 0;
-                payload[bytesWritten++] = 0;
+                Span<byte> iTxtHeader = payload.Slice(bytesWritten);
+                iTxtHeader[4] = 0;
+                iTxtHeader[3] = 0;
+                iTxtHeader[2] = 0;
+                iTxtHeader[1] = 0;
+                iTxtHeader[0] = 0;
+                bytesWritten += 5;
 
                 // And the XMP data itself.
                 xmpData.CopyTo(payload.Slice(bytesWritten));