Cast to long and add test/guard.

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

@@ -156,7 +156,7 @@ internal Memory<T> GetSafeRowMemory(int y)
         {
             DebugGuard.MustBeGreaterThanOrEqualTo(y, 0, nameof(y));
             DebugGuard.MustBeLessThan(y, this.Height, nameof(y));
-            return this.FastMemoryGroup.View.GetBoundedSlice(y * this.Width, this.Width);
+            return this.FastMemoryGroup.View.GetBoundedSlice(y * (long)this.Width, this.Width);
         }
 
         /// <summary>
@@ -200,7 +200,7 @@ internal static void SwapOrCopyContent(Buffer2D<T> destination, Buffer2D<T> sour
         }
 
         [MethodImpl(InliningOptions.ColdPath)]
-        private Memory<T> GetRowMemorySlow(int y) => this.FastMemoryGroup.GetBoundedSlice(y * this.Width, this.Width);
+        private Memory<T> GetRowMemorySlow(int y) => this.FastMemoryGroup.GetBoundedSlice(y * (long)this.Width, this.Width);
 
         [MethodImpl(InliningOptions.ColdPath)]
         private Memory<T> GetSingleMemorySlow() => this.FastMemoryGroup.Single();

src/ImageSharp/Memory/DiscontiguousBuffers/MemoryGroupExtensions.cs

@@ -1,4 +1,4 @@
-// Copyright (c) Six Labors and contributors.
+// Copyright (c) Six Labors and contributors.
 // Licensed under the Apache License, Version 2.0.
 
 using System;
@@ -38,6 +38,12 @@ internal static Memory<T> GetBoundedSlice<T>(this IMemoryGroup<T> group, long st
             Guard.MustBeLessThan(start, group.TotalLength, nameof(start));
 
             int bufferIdx = (int)(start / group.BufferLength);
+
+            if (bufferIdx < 0)
+            {
+                throw new ArgumentOutOfRangeException(nameof(start));
+            }
+
             if (bufferIdx >= group.Count)
             {
                 throw new ArgumentOutOfRangeException(nameof(start));

tests/ImageSharp.Tests/Memory/DiscontiguousBuffers/MemoryGroupTests.cs

@@ -152,6 +152,7 @@ public void GetBoundedSlice_WhenArgsAreCorrect(long totalLength, int bufferLengt
 
         public static TheoryData<long, int, long, int> GetBoundedSlice_ErrorData = new TheoryData<long, int, long, int>()
         {
+            { 300, 100, -1, 91 },
             { 300, 100, 110, 91 },
             { 42, 7, 0, 8 },
             { 42, 7, 1, 7 },