httppeer: make __del__ access to self.urlopener more safe

Some errors could in some cases show unfortunate scary and confusing warnings
from the httppeer delstructors:

  abort: nodename nor servname provided, or not known
  Exception AttributeError: "'httpspeer' object has no attribute 'urlopener'" in <bound method httpspeer.__del__ of <mercurial.httppeer.httpspeer object at 0x106e1f5d0>> ignored```

To mute that, take 7b15dd9125b3 to the next level and use getattr in __del__.

mercurial/httppeer.py

@@ -63,8 +63,9 b' class httppeer(wireproto.wirepeer):'
         self.requestbuilder = urlreq.request
 
     def __del__(self):
-        if self.urlopener:
-            for h in self.urlopener.handlers:
+        urlopener = getattr(self, 'urlopener', None)
+        if urlopener:
+            for h in urlopener.handlers:
                 h.close()
                 getattr(h, "close_all", lambda : None)()