Fix IOStreamStreamReader to account for alignment padding

[josalem]

When reading a non-seekable stream, the IOStreamStreamReader class can be put in a state where it attempts to read a count greater than length - offset of its internal buffer.

simple repro

class Program
    {
        public static int align = 8;
        public static int size = 0x4000;

        static void Main(string[] args)
        {
            using var memoryStream = new MemoryStream();

            for (int i = 0; i < 20 * size; i++)
            {
                memoryStream.WriteByte(0xFD);
            }

            memoryStream.Seek(0, SeekOrigin.Begin);
            var nonSeekableStream = new NonSeekableStream(memoryStream);

            var dst = new byte[size * 2];
            using var streamReader = new IOStreamStreamReader(nonSeekableStream);

            // Put the stream reader in a bad state
            streamReader.Read(dst, 0, 14); // Read a length in (8,16)

            streamReader.Read(dst, 0, size + align); // read a length in (size, size + align]
        }
    }

This happens because the constructor for the class pads the buffer size by align bytes so that it has room to round up the number of bytes read to an aligned value. The reading logic doesn't preserve the padding and uses the requestedSize + align length for some of its logic which is how we get into this corner case. It is a rare occurrence, but this has been what is causing CI failures in CoreCLR (see: dotnet/coreclr#26241).

I'm going to be testing this locally with EventPipe workloads (a main use case for this class).

CC - @tommcdon, @brianrob, @noahfalk, @sywhang