Fix 'NoneType' object has no attribute 'can_read' (#1107)

The root cause of the issue is that the connection
can be erased during an execution of self._receive_one(c).
So c.connection should be checked before every iteration,
not only before the first one.

kombu/transport/redis.py

@@ -684,9 +684,8 @@ class Channel(virtual.Channel):
             ret.append(self._receive_one(c))
         except Empty:
             pass
-        if c.connection is not None:
-            while c.connection.can_read(timeout=0):
-                ret.append(self._receive_one(c))
+        while c.connection is not None and c.connection.can_read(timeout=0):
+            ret.append(self._receive_one(c))
         return any(ret)
 
     def _receive_one(self, c):

t/unit/transport/test_redis.py

@@ -807,6 +807,18 @@ class TestRedisChannel(unittest.TestCase):
 
         assert self.channel._receive_one(self.channel.subclient) is None
 
+    def test_receive_connection_has_gone(self):
+        def _receive_one(c):
+            c.connection = None
+            _receive_one.called = True
+            return True
+
+        _receive_one.called = False
+        self.channel._receive_one = _receive_one
+
+        assert self.channel._receive()
+        assert _receive_one.called
+
     def test_brpop_read_raises(self):
         c = self.channel.client = Mock()
         c.parse_response.side_effect = KeyError('foo')