Fixed BufferedStream.ReadByteSlow() to correctly set _readPos (#1)

The following code throws an exception, caused by incorrect logic in BufferedStream.ReadByteSlow():

            byte[] input = new byte[] { 1, 2 };
            using (var reader = new BufferedStream(new MemoryStream(input), 2))
            {
                reader.ReadByte();
                reader.ReadByte();
                reader.ReadByte();

                byte[] mybuffer = new byte[10];
                reader.Read(mybuffer, 0, 1);   // throws System.ArgumentOutOfRangeException: 'Non-negative number required.'
            }

When the input data is exhausted through a sequence of ReadByte() calls, ReadByteSlow() does not reset _readPos to zero.
This causes a break in the contract specified in Read() - because because _readLen - _readPos is negative.
Then the block copy fails because the count to copy is negative.
Fix is to reset _readPos to zero before returning -1.

src/System.Runtime.Extensions/src/System/IO/BufferedStream.cs

@@ -690,10 +690,11 @@ private int ReadByteSlow()
 
  EnsureBufferAllocated();
  _readLen = _stream.Read(_buffer, 0, _bufferSize);
+ _readPos = 0;
+
  if (_readLen == 0)
  return -1;
 
- _readPos = 0;
  return _buffer[_readPos++];
  }