proc: don't print erroneous traceback for cancelled links

gevent/proc.py

@@ -148,8 +148,7 @@ class Link(object):
 class LinkToEvent(Link):
 
     def __call__(self, source):
-        if self.listener is None:
-            return
+        if self.listener is not None:
             if source.has_value():
                 self.listener.send(source.value)
             else:
@@ -158,6 +157,7 @@ class LinkToEvent(Link):
 class LinkToGreenlet(Link):
 
     def __call__(self, source):
+        if self.listener is not None:
             if source.has_value():
                 self.listener.throw(LinkedCompleted(source.name))
             else:
@@ -166,6 +166,7 @@ class LinkToGreenlet(Link):
 class LinkToCallable(Link):
 
     def __call__(self, source):
+        if self.listener is not None:
             self.listener(source)