frame: fix AttributeError on python 3.5+

Protocol.reader in Python3.5 is set to None by the connection_lost
method.  Occasionally, this leads to endless exception throws.  This fix
may close issue #115.

aioamqp/frame.py

@@ -404,6 +404,8 @@ def __init__(self, reader):
     @asyncio.coroutine
     def read_frame(self):
         """Decode the frame"""
+        if not self.reader:
+            raise exceptions.AmqpClosedConnection()
         try:
             data = yield from self.reader.readexactly(7)
         except (asyncio.IncompleteReadError, socket.error) as ex: