use ImageSharp Guard / DebugGuard

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

@@ -62,7 +62,7 @@ namespace SixLabors.ImageSharp.Memory
             get
             {
                 ImageSharp.DebugGuard.MustBeLessThan(x, this.Width, nameof(x));
-                SixLabors.DebugGuard.MustBeLessThan(y, this.Height, nameof(y));
+                ImageSharp.DebugGuard.MustBeLessThan(y, this.Height, nameof(y));
                 Span<T> span = this.Span;
                 return ref span[(this.Width * y) + x];
             }

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

@@ -123,7 +123,7 @@ namespace SixLabors.ImageSharp.Memory
         public BufferArea<T> GetSubArea(Rectangle rectangle)
         {
             ImageSharp.DebugGuard.MustBeLessThanOrEqualTo(rectangle.Width, this.Rectangle.Width, nameof(rectangle));
-            SixLabors.DebugGuard.MustBeLessThanOrEqualTo(rectangle.Height, this.Rectangle.Height, nameof(rectangle));
+            ImageSharp.DebugGuard.MustBeLessThanOrEqualTo(rectangle.Height, this.Rectangle.Height, nameof(rectangle));
 
             int x = this.Rectangle.X + rectangle.X;
             int y = this.Rectangle.Y + rectangle.Y;