Make releaseconnection a NOOP when conn is None
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 <98dcb2717ddae152d5b359c6ea97e4fe34a29d4c@SSpaeth.de>

frioux/offlineimap,frioux/offlineimap