try to ensure we only call the close callback once

git-svn-id: https://xpra.org/svn/Xpra/trunk@8906 3bb7dfac-3a0b-4e04-842a-767bc560f471

src/xpra/net/bytestreams.py

@@ -175,8 +175,10 @@ def close(self):
             self._writeable.close()
         except:
             pass
-        if self._close_cb:
-            self._close_cb()
+        cc = self._close_cb
+        if cc:
+            self._close_cb = None
+            cc()
 
     def __repr__(self):
         return "TwoFileConnection(%s)" % str(self.target)