Disallow allocation attempts of unrepresentable sizes

backport of #2545

src/ImageSharp/Memory/Allocators/UniformUnmanagedMemoryPoolMemoryAllocator.cs

@@ -121,6 +121,11 @@ namespace SixLabors.ImageSharp.Memory
             AllocationOptions options = AllocationOptions.None)
         {
             long totalLengthInBytes = totalLength * Unsafe.SizeOf<T>();
+            if (totalLengthInBytes < 0)
+            {
+                throw new InvalidMemoryOperationException("Attempted to allocate a MemoryGroup of a size that is not representable.");
+            }
+
             if (totalLengthInBytes <= this.sharedArrayPoolThresholdInBytes)
             {
                 var buffer = new SharedArrayPoolBuffer<T>((int)totalLength);

tests/ImageSharp.Tests/Image/ImageTests.cs

@@ -40,6 +40,10 @@ namespace SixLabors.ImageSharp.Tests
                 }
             }
 
+            [Fact]
+            public void Width_Height_SizeNotRepresentable_ThrowsInvalidImageOperationException()
+                => Assert.Throws<InvalidMemoryOperationException>(() => new Image<Rgba32>(int.MaxValue, int.MaxValue));
+
             [Fact]
             public void Configuration_Width_Height()
             {

tests/ImageSharp.Tests/Memory/Allocators/UniformUnmanagedPoolMemoryAllocatorTests.cs

@@ -111,6 +111,13 @@ namespace SixLabors.ImageSharp.Tests.Memory.Allocators
             }
         }
 
+        [Fact]
+        public void AllocateGroup_SizeInBytesOverLongMaxValue_ThrowsInvalidMemoryOperationException()
+        {
+            var allocator = new UniformUnmanagedMemoryPoolMemoryAllocator(null);
+            Assert.Throws<InvalidMemoryOperationException>(() => allocator.AllocateGroup<S4>(int.MaxValue * (long)int.MaxValue, int.MaxValue));
+        }
+
         [Fact]
         public unsafe void Allocate_MemoryIsPinnableMultipleTimes()
         {