Make sure that _read_watcher is not empty before start it

Prevent following to happen:
```
  File "cassandra/io/libevreactor.py", line 197, in _loop_will_run
    conn._read_watcher.start()
AttributeError: 'NoneType' object has no attribute 'start'
```

Author: dkropachev

cassandra/io/libevreactor.py

@@ -194,7 +194,8 @@ def _loop_will_run(self, prepare):
                 self._new_conns = set()
 
             for conn in to_start:
-                conn._read_watcher.start()
+                if conn._read_watcher:
+                    conn._read_watcher.start()
 
             changed = True