Remove unnecessary throw and optimize write.

src/ImageSharp/Formats/Gif/GifEncoderCore.cs

@@ -428,26 +428,31 @@ namespace SixLabors.ImageSharp.Formats.Gif
             where TGifExtension : struct, IGifExtension
         {
             IMemoryOwner<byte> owner = null;
-            Span<byte> buffer;
+            Span<byte> extensionBuffer;
             int extensionSize = extension.ContentLength;
-            if (extensionSize > this.buffer.Length - 3)
+
+            if (extensionSize == 0)
+            {
+                return;
+            }
+            else if (extensionSize > this.buffer.Length - 3)
             {
                 owner = this.memoryAllocator.Allocate<byte>(extensionSize + 3);
-                buffer = owner.GetSpan();
+                extensionBuffer = owner.GetSpan();
             }
             else
             {
-                buffer = this.buffer;
+                extensionBuffer = this.buffer;
             }
 
-            buffer[0] = GifConstants.ExtensionIntroducer;
-            buffer[1] = extension.Label;
+            extensionBuffer[0] = GifConstants.ExtensionIntroducer;
+            extensionBuffer[1] = extension.Label;
 
-            extension.WriteTo(buffer.Slice(2));
+            extension.WriteTo(extensionBuffer.Slice(2));
 
-            buffer[extensionSize + 2] = GifConstants.Terminator;
+            extensionBuffer[extensionSize + 2] = GifConstants.Terminator;
 
-            stream.Write(buffer, 0, extensionSize + 3);
+            stream.Write(extensionBuffer, 0, extensionSize + 3);
             owner?.Dispose();
         }
 

src/ImageSharp/Formats/Gif/Sections/GifGraphicControlExtension.cs

@@ -71,13 +71,11 @@ namespace SixLabors.ImageSharp.Formats.Gif
 
             dest = this;
 
-            return 5;
+            return ((IGifExtension)this).ContentLength;
         }
 
         public static GifGraphicControlExtension Parse(ReadOnlySpan<byte> buffer)
-        {
-            return MemoryMarshal.Cast<byte, GifGraphicControlExtension>(buffer)[0];
-        }
+            => MemoryMarshal.Cast<byte, GifGraphicControlExtension>(buffer)[0];
 
         public static byte GetPackedValue(GifDisposalMethod disposalMethod, bool userInputFlag = false, bool transparencyFlag = false)
         {

src/ImageSharp/Formats/Gif/Sections/GifNetscapeLoopingApplicationExtension.cs

@@ -40,7 +40,7 @@ namespace SixLabors.ImageSharp.Formats.Gif
             // 0 means loop indefinitely. Count is set as play n + 1 times.
             BinaryPrimitives.WriteUInt16LittleEndian(buffer.Slice(14, 2), this.RepeatCount);
 
-            return 16; // Length - Introducer + Label + Terminator.
+            return this.ContentLength; // Length - Introducer + Label + Terminator.
         }
     }
 }

src/ImageSharp/Formats/Gif/Sections/GifXmpApplicationExtension.cs

@@ -17,7 +17,7 @@ namespace SixLabors.ImageSharp.Formats.Gif
         // size          : 1
         // identifier    : 11
         // magic trailer : 257
-        public int ContentLength => this.Data.Length + 269;
+        public int ContentLength => (this.Data.Length > 0) ? this.Data.Length + 269 : 0;
 
         /// <summary>
         /// Gets the raw Data.
@@ -50,12 +50,6 @@ namespace SixLabors.ImageSharp.Formats.Gif
 
         public int WriteTo(Span<byte> buffer)
         {
-            int totalSize = this.ContentLength;
-            if (buffer.Length < totalSize)
-            {
-                ThrowInsufficientMemory();
-            }
-
             int bytesWritten = 0;
             buffer[bytesWritten++] = GifConstants.ApplicationBlockSize;
 
@@ -77,7 +71,7 @@ namespace SixLabors.ImageSharp.Formats.Gif
 
             buffer[bytesWritten++] = 0x00;
 
-            return totalSize;
+            return this.ContentLength;
         }
 
         private static byte[] ReadXmpData(Stream stream, MemoryAllocator allocator)
@@ -100,8 +94,5 @@ namespace SixLabors.ImageSharp.Formats.Gif
                 bytes.WriteByte((byte)b);
             }
         }
-
-        private static void ThrowInsufficientMemory()
-            => throw new InsufficientMemoryException("Unable to write XMP metadata to GIF image");
     }
 }