Fix review feedback

Author: JimBobSquarePants

src/ImageSharp/Image.WrapMemory.cs

@@ -609,14 +609,14 @@ public static unsafe Image<TPixel> WrapMemory<TPixel>(
         ImageMetadata metadata)
         where TPixel : unmanaged, IPixel<TPixel>
     {
-        Guard.IsFalse(pointer == null, nameof(pointer), "Pointer must be not null");
+        Guard.IsFalse(pointer == null, nameof(pointer), "Pointer must not be null");
         Guard.NotNull(configuration, nameof(configuration));
         Guard.NotNull(metadata, nameof(metadata));
 
         int rowStride = GetPixelRowStrideFromByteStride<TPixel>(width, rowStrideInBytes, nameof(rowStrideInBytes));
         long requiredLength = GetRequiredLength(width, height, rowStride);
 
-        Guard.MustBeLessThanOrEqualTo(requiredLength, int.MaxValue, nameof(height));
+        Guard.MustBeLessThanOrEqualTo(requiredLength, int.MaxValue, nameof(requiredLength));
         Guard.MustBeGreaterThanOrEqualTo(bufferSizeInBytes / Unsafe.SizeOf<TPixel>(), requiredLength, nameof(bufferSizeInBytes));
 
         UnmanagedMemoryManager<TPixel> memoryManager = new(pointer, (int)requiredLength);

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

@@ -356,7 +356,7 @@ internal Memory<T> GetSafeRowMemory(int y)
     /// Thrown when the backing group is discontiguous.
     /// </exception>
     [MethodImpl(InliningOptions.ShortMethod)]
-    internal Span<T> DangerousGetSingleSpan() => this.FastMemoryGroup[0].Span;
+    internal Span<T> DangerousGetSingleSpan() => this.FastMemoryGroup.Single().Span;
 
     /// <summary>
     /// Gets a <see cref="Memory{T}"/> to the backing data of if the backing group consists of a single contiguous memory buffer.
@@ -367,7 +367,7 @@ internal Memory<T> GetSafeRowMemory(int y)
     /// Thrown when the backing group is discontiguous.
     /// </exception>
     [MethodImpl(InliningOptions.ShortMethod)]
-    internal Memory<T> DangerousGetSingleMemory() => this.FastMemoryGroup[0];
+    internal Memory<T> DangerousGetSingleMemory() => this.FastMemoryGroup.Single();
 
     /// <summary>
     /// Swaps the contents of 'destination' with 'source' if the buffers are owned (1),
@@ -414,5 +414,5 @@ private void ThrowYOutOfRangeException(int y)
 
     [MethodImpl(InliningOptions.ShortMethod)]
     private static long GetRequiredLength(int width, int height, int stride)
-        => ((long)(height - 1) * stride) + width;
+        => checked(((long)(height - 1) * stride) + width);
 }