make reinit() and fork() only call loop.reinit() if there's a hub

gevent/__init__.py

@@ -41,16 +41,12 @@ from gevent.greenlet import Greenlet, joinall, killall
 spawn = Greenlet.spawn
 spawn_later = Greenlet.spawn_later
 from gevent.timeout import Timeout, with_timeout
-from gevent.hub import getcurrent, GreenletExit, spawn_raw, sleep, idle, kill, signal
+from gevent.hub import getcurrent, GreenletExit, spawn_raw, sleep, idle, kill, signal, reinit
 try:
     from gevent.hub import fork
 except ImportError:
     __all__.remove('fork')
 
 
-def reinit():
-    return get_hub().loop.reinit()
-
-
 def run(timeout=None, event=None):
     return get_hub().join(timeout=timeout, event=event)

gevent/hub.py

@@ -18,6 +18,7 @@ __all__ = ['getcurrent',
            'sleep',
            'kill',
            'signal',
+           'reinit',
            'fork',
            'get_hub',
            'Hub',
@@ -176,12 +177,18 @@ class signal(object):
             self.hub.handle_error(None, *sys.exc_info())
 
 
+def reinit():
+    hub = _get_hub()
+    if hub is not None:
+        hub.loop.reinit()
+
+
 if _original_fork is not None:
 
     def fork():
         result = _original_fork()
         if not result:
-            get_hub().loop.reinit()
+            reinit()
         return result