NpipeSocket.makefile crashes when bufsize < 0
**Original report**: https://github.com/docker/compose/issues/3901#issuecomment-244828701

Negative `bufsize` should be handled.

diff --git a/docker/transport/npipesocket.py b/docker/transport/npipesocket.py
index 35418ef1..9010cebe 100644
--- a/docker/transport/npipesocket.py
+++ b/docker/transport/npipesocket.py
@@ -94,7 +94,7 @@ class NpipeSocket(object):
         if mode.strip('b') != 'r':
             raise NotImplementedError()
         rawio = NpipeFileIOBase(self)
-        if bufsize is None:
+        if bufsize is None or bufsize < 0:
             bufsize = io.DEFAULT_BUFFER_SIZE
         return io.BufferedReader(rawio, buffer_size=bufsize)
 
['docker/transport/npipesocket.py']