Returning None where messagelist dict expected

[sigma914]

I've found and error in v6.5.7-rc2

This seems to be caused by setting messagelist to None to invalidate a cache and then not checking it in various places.

ERROR: ERROR in syncfolder for <snip> folder Sent: Traceback (most recent call last):
  File "/usr/lib/python2.7/site-packages/offlineimap/accounts.py", line 466, in syncfolder
    remotefolder.syncmessagesto(localfolder, statusfolder)
  File "/usr/lib/python2.7/site-packages/offlineimap/folder/Base.py", line 893, in syncmessagesto
    action(dstfolder, statusfolder)
  File "/usr/lib/python2.7/site-packages/offlineimap/folder/Base.py", line 787, in __syncmessagesto_delete
    statusfolder.getmessageuidlist())
  File "/usr/lib/python2.7/site-packages/offlineimap/folder/Base.py", line 277, in getmessageuidlist
    return self.getmessagelist().keys()
AttributeError: 'NoneType' object has no attribute 'keys'

[nicolas33]

Fixed in 9e63fa3.