folder/IMAP: Remove buggy duplicate assignment

we do:
  for msgid in imapdata:
      maxmsgid = max(long(msgid), maxmsgid)
and then basically immediately:
   maxmsgid = long(imapdata[0])

throwing away the first assignment although the first method of
assigning is the correct one. The second had been forgotten to be
removed when we introduced the above iteration. This bug would fix a
regression with those broken ZIMBRA servers that send multiple EXISTS
replies.

Signed-off-by: Sebastian Spaeth <Sebastian@SSpaeth.de>
Signed-off-by: Nicolas Sebrecht <nicolas.s-dev@laposte.net>

offlineimap/folder/IMAP.py

@@ -164,13 +164,11 @@ class IMAPFolder(BaseFolder):
                 maxmsgid = 0
                 for msgid in imapdata:
                     maxmsgid = max(long(msgid), maxmsgid)
-
-                maxmsgid = long(imapdata[0])
-                messagesToFetch = '1:%d' % maxmsgid;
-
                 if maxmsgid < 1:
                     #no messages; return
                     return
+                messagesToFetch = '1:%d' % maxmsgid;
+
             # Now, get the flags and UIDs for these.
             # We could conceivably get rid of maxmsgid and just say
             # '1:*' here.