Make releaseconnection a NOOP when conn is None

During cleanup we often call releaseconnection in a finally: block. But
in cases of error, we might have dropped the connection earlier already
and set it to "None". In this case don't fail releaseconnection() but
make it a NOOP.

Signed-off-by: Sebastian Spaeth <Sebastian@SSpaeth.de>

offlineimap/imapserver.py

@@ -113,6 +113,7 @@ def releaseconnection(self, connection, drop_conn=False):
 
         :param drop_conn: If True, the connection will be released and
            not be reused. This can be used to indicate broken connections."""
+        if connection is None: return #noop on bad connection
         self.connectionlock.acquire()
         self.assignedconnections.remove(connection)
         # Don't reuse broken connections