Attempt to fix a crashing bug in Gmail driver

refs deb#469598

Bug report received in Debian.  User is having occasional trouble with Gmail
driver crashing, indicates problem may have started after switching to Gmail.

Gmail driver merged to tree in 81b86fb74 on 2008-01-03 and recently released
in beta tarballs.

BT shows:

  File "/var/lib/python-support/python2.4/offlineimap/folder/Gmail.py", line 102, in processmessagesflags
    attributehash = imaputil.flags2hash(imaputil.imapsplit(result)[1])
  File "/var/lib/python-support/python2.4/offlineimap/imaputil.py", line 88, in imapsplit
    for i in range(len(imapstring)):
TypeError: len() of unsized object

imap debug log shows:

imap:   29:58.16 > BEAL75 UID STORE 4887 +FLAGS (\Deleted)
imap:   29:58.47 < BEAL75 OK Success
imap:   29:58.47        matched r'(?P<tag>BEAL\d+) (?P<type>[A-Z]+) (?P<data>.*)' => ('BEAL75', 'OK', 'Success')
imap: imapsplit() called with input: None
imap: imapsplit() got a non-string input; working around.

looking at code for Gmail.py processmessagesflags, comments from Ricardo
indicate it was copied from the same function in folder/IMAP.py.

However, folder/IMAP.py has checks for this, added 2002-07-12 in
5342dacc & 817a10ce.  Suspect that Gmail author believed those checks
superfluous for Gmail.

Copied checks from folder/IMAP.py to folder/Gmail.py.

offlineimap/folder/Gmail.py

@@ -99,7 +99,14 @@ class GmailFolder(IMAPFolder):
 
         needupdate = copy(uidlist)
         for result in r:
+            if result == None:
+                # Compensate for servers that don't return anything from
+                # STORE.
+                continue
             attributehash = imaputil.flags2hash(imaputil.imapsplit(result)[1])
+            if not ('UID' in attributehash and 'FLAGS' in attributehash):
+                # Compensate for servers that don't return a UID attribute.
+                continue
             flags = attributehash['FLAGS']
             uid = long(attributehash['UID'])
             self.messagelist[uid]['flags'] = imaputil.flagsimap2maildir(flags)