Fix overflow in MemoryAllocator.Create(options) (#2730)

Fix an overlook from #2706. See 92b8277#r141770676.
# Conflicts:
#	src/ImageSharp/Memory/Allocators/MemoryAllocator.cs

Author: antonfirsov

src/ImageSharp/Memory/Allocators/MemoryAllocator.cs

@@ -51,7 +51,7 @@ public static MemoryAllocator Create(MemoryAllocatorOptions options)
             UniformUnmanagedMemoryPoolMemoryAllocator allocator = new(options.MaximumPoolSizeMegabytes);
             if (options.AllocationLimitMegabytes.HasValue)
             {
-                allocator.MemoryGroupAllocationLimitBytes = options.AllocationLimitMegabytes.Value * 1024 * 1024;
+                allocator.MemoryGroupAllocationLimitBytes = options.AllocationLimitMegabytes.Value * 1024L * 1024L;
                 allocator.SingleBufferAllocationLimitBytes = (int)Math.Min(allocator.SingleBufferAllocationLimitBytes, allocator.MemoryGroupAllocationLimitBytes);
             }
 

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

@@ -436,5 +436,21 @@ static void RunTest()
             }
         }
 #endif
+
+        [ConditionalFact(typeof(Environment), nameof(Environment.Is64BitProcess))]
+        public void MemoryAllocator_Create_SetHighLimit()
+        {
+            RemoteExecutor.Invoke(RunTest).Dispose();
+            static void RunTest()
+            {
+                const long threeGB = 3L * (1 << 30);
+                MemoryAllocator allocator = MemoryAllocator.Create(new MemoryAllocatorOptions()
+                {
+                    AllocationLimitMegabytes = (int)(threeGB / 1024)
+                });
+                using MemoryGroup<byte> memoryGroup = allocator.AllocateGroup<byte>(threeGB, 1024);
+                Assert.Equal(threeGB, memoryGroup.TotalLength);
+            }
+        }
     }
 }