libev reactor: Return from watchers for closed connection

Previous commit defered socket close until watchers are stopped, but
there is one more case worth considering.
If during one libev loop iteration socket gets ready for both read and
write, then both watchers will be called. If one decides to close the
connection, the other one will still get called anyway.
This shouldn't cause EBADF, because socket won't be closed yet, but I
see no reason to perform unnecessary work.

Author: Lorak-mmk

cassandra/io/libevreactor.py

@@ -321,6 +321,8 @@ def close(self):
             self.connected_event.set()
 
     def handle_write(self, watcher, revents, errno=None):
+        if self.is_closed:
+            return
         if revents & libev.EV_ERROR:
             if errno:
                 exc = IOError(errno, os.strerror(errno))
@@ -362,6 +364,8 @@ def handle_write(self, watcher, revents, errno=None):
                         return
 
     def handle_read(self, watcher, revents, errno=None):
+        if self.is_closed:
+            return
         if revents & libev.EV_ERROR:
             if errno:
                 exc = IOError(errno, os.strerror(errno))