fix memory sentinel initialization

src/ImageSharp/Memory/Allocators/Internals/UnmanagedBuffer{T}.cs

@@ -32,6 +32,7 @@ namespace SixLabors.ImageSharp.Memory.Internals
         {
             this.lengthInElements = lengthInElements;
             this.BufferHandle = bufferHandle;
+            this.BufferHandle.InitMemorySentinel();
         }
 
         public UnmanagedMemoryHandle BufferHandle { get; protected set; }
@@ -71,6 +72,6 @@ namespace SixLabors.ImageSharp.Memory.Internals
         }
 
         [Conditional("MEMORY_SENTINEL")]
-        protected void VerifyMemorySentinel() => this.BufferHandle.VerifyMemorySentinel(this.lengthInElements * Unsafe.SizeOf<T>());
+        internal void VerifyMemorySentinel() => this.BufferHandle.VerifyMemorySentinel(this.lengthInElements * Unsafe.SizeOf<T>());
     }
 }

src/ImageSharp/Memory/Allocators/Internals/UnmanagedMemoryHandle.cs

@@ -85,15 +85,12 @@ namespace SixLabors.ImageSharp.Memory.Internals
         internal static UnmanagedMemoryHandle Allocate(int lengthInBytes)
         {
             IntPtr handle = AllocateHandle(lengthInBytes + MemorySentinelPadding);
-#if MEMORY_SENTINEL
-            unsafe
-            {
-                new Span<byte>((void*)handle, lengthInBytes + MemorySentinelPadding).Fill(42);
-            }
-#endif
             return new UnmanagedMemoryHandle(handle, lengthInBytes);
         }
 
+        [Conditional("MEMORY_SENTINEL")]
+        internal unsafe void InitMemorySentinel() => new Span<byte>((void*)this.handle, this.lengthInBytes + MemorySentinelPadding).Fill(42);
+
         [Conditional("MEMORY_SENTINEL")]
         internal unsafe void VerifyMemorySentinel(int actualLengthInBytes)
         {

src/ImageSharp/Memory/Buffer2D{T}.cs

@@ -164,6 +164,8 @@ namespace SixLabors.ImageSharp.Memory
         [MethodImpl(InliningOptions.ShortMethod)]
         internal Memory<T> DangerousGetSingleMemory() => this.FastMemoryGroup.Single();
 
+        internal void VerifyMemorySentinel() => this.FastMemoryGroup.VerifyMemorySentinel();
+
         /// <summary>
         /// Swaps the contents of 'destination' with 'source' if the buffers are owned (1),
         /// copies the contents of 'source' to 'destination' otherwise (2). Buffers should be of same size in case 2!

src/ImageSharp/Memory/DiscontiguousBuffers/MemoryGroup{T}.Owned.cs

@@ -111,6 +111,17 @@ namespace SixLabors.ImageSharp.Memory
                 }
             }
 
+            public override void VerifyMemorySentinel()
+            {
+                foreach (IMemoryOwner<T> memoryOwner in this.memoryOwners)
+                {
+                    if (memoryOwner is UnmanagedBuffer<T> unmanagedBuffer)
+                    {
+                        unmanagedBuffer.VerifyMemorySentinel();
+                    }
+                }
+            }
+
             /// <inheritdoc/>
             IEnumerator<Memory<T>> IEnumerable<Memory<T>>.GetEnumerator()
             {

src/ImageSharp/Memory/DiscontiguousBuffers/MemoryGroup{T}.cs

@@ -278,5 +278,9 @@ namespace SixLabors.ImageSharp.Memory
         public virtual void DecreaseRefCounts()
         {
         }
+
+        public virtual void VerifyMemorySentinel()
+        {
+        }
     }
 }