Fix max recursion depth in GreenSocket.__del__

I'm happy to make you happy. :)

```
% cat t.py                                                                                                        :(
def test_foo():
    d = {'foo': 'bar'}
    k = 'foo'
    assert d[k] == k
temoto@shepelev-ayyo ~/dev/github.com/eventlet/eventlet (git)-[master] % venv-27/bin/nosetests t.py   
F
======================================================================
FAIL: t.test_foo
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/temoto/dev/github.com/eventlet/eventlet/venv-27/local/lib/python2.7/site-packages/nose/case.py", line 197, in runTest
    self.test(*self.arg)
  File "/home/temoto/dev/github.com/eventlet/eventlet/t.py", line 4, in test_foo
    assert d[k] == k
AssertionError

----------------------------------------------------------------------
Ran 1 test in 0.000s

FAILED (failures=1)
1 temoto@shepelev-ayyo ~/dev/github.com/eventlet/eventlet (git)-[master] % venv-27/bin/nosetests -d t.py                                                                                   :(
F
======================================================================
FAIL: t.test_foo
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/temoto/dev/github.com/eventlet/eventlet/venv-27/local/lib/python2.7/site-packages/nose/case.py", line 197, in runTest
    self.test(*self.arg)
  File "/home/temoto/dev/github.com/eventlet/eventlet/t.py", line 4, in test_foo
    assert d[k] == k
AssertionError: 
    {'foo': 'bar'} = {'foo': 'bar'}
    'foo' = 'foo'
>>  assert {'foo': 'bar'}['foo'] == 'foo'
    

----------------------------------------------------------------------
Ran 1 test in 0.009s

FAILED (failures=1)
```