Fix double release of IMAP connection object

This commit fixes the case when we're invoking releaseconnection()
for a given imapobj twice.

This bug manifests itself as
{{{
ValueError: list.remove(x): x not in list

  File "[...]/offlineimap/folder/IMAP.py", line 615, in savemessage
    self.imapserver.releaseconnection(imapobj)
  File "[...]/offlineimap/imapserver.py", line 130, in releaseconnection
    self.assignedconnections.remove(connection)
}}}

Signed-off-by: Eygene Ryabinkin <rea@codelabs.ru>