runtime: SequenceReader.TryCopyMultisegment has incorrect bounds check

The problematic code: https://github.com/dotnet/corefx/blob/8292e4556eae08f497e3d8666c001b5ea75f0d3b/src/System.Memory/src/System/Buffers/SequenceReader.cs#L346-L347

It looks like it’s meant to be a guard to ensure there is enough buffer to copy the remaining from the unread span, but it’s inverted. When working with a multi-span sequence, if the provided buffer is larger than the remaining code, SequenceReader.TryCopyTo will always fail

  class Program
    {
        static void Main(string[] args)
        {
            var segment = new BufferSegment<byte>(new[] { (byte)1, (byte)'A' });
            segment.Append(new[] { (byte)'B'});
            var sequence = new ReadOnlySequence<byte>(segment, 0, segment.Next, 1);

            var sequenceReader = new SequenceReader<byte>(sequence);
            sequenceReader.TryRead(out var value);
            Debug.Assert(value == (byte)1);

            var readBuffer = new byte[16];
            var result = sequenceReader.TryCopyTo(readBuffer);
            System.Console.WriteLine($"{result} {readBuffer[0]} {readBuffer[1]}");

            readBuffer = new byte[2];
            result = sequenceReader.TryCopyTo(readBuffer);
            System.Console.WriteLine($"{result} {readBuffer[0]} {readBuffer[1]}");
        }

        internal class BufferSegment<T> : ReadOnlySequenceSegment<T>
        {
            public BufferSegment(ReadOnlyMemory<T> memory)
            {
                Memory = memory;
            }

            public BufferSegment<T> Append(ReadOnlyMemory<T> memory)
            {
                var segment = new BufferSegment<T>(memory)
                {
                    RunningIndex = RunningIndex + Memory.Length
                };
                Next = segment;
                return segment;
            }
        }
    }

Output:

False 0 0
True 65 66

Environment: Microsoft.NETCore.App 3.0.0-preview5-27619-16