Do not try to release IMAP connection twice

Reported by sharat87 in https://github.com/spaetz/offlineimap/pull/38,
he would often get an unhandled Exception when trying to
releaseconnection() a connection that was not in the pool of
connections.

The reason this could happen is that when folder.IMAP.quickchanged()
raises an Exception in select(), we would release the connection in the
"except" handling, and than release the same connection in the "finally"
clause, which led to the error. The right thing is to only release the
connection once, of course.

Signed-off-by: Sebastian Spaeth <Sebastian@SSpaeth.de>

Changelog.rst

@@ -12,6 +12,7 @@ WIP (add new stuff for the next release)
 * Actually perform the SSL fingerprint check (reported by J. Cook)
 * Curses UI, don't use colors after we shut down curses already (C.Höger)
 * Document that '%' needs encoding as '%%' in *.conf
+* Fix crash when IMAP.quickchanged() led to an Error (reported by sharat87)
 
 OfflineIMAP v6.5.3.1 (2012-04-03)
 =================================

offlineimap/folder/IMAP.py

@@ -93,14 +93,17 @@ class IMAPFolder(BaseFolder):
                 # Select folder and get number of messages
                 restype, imapdata = imapobj.select(self.getfullname(), True,
                                                    True)
+                self.imapserver.releaseconnection(imapobj)
             except OfflineImapError as e:
                 # retry on dropped connections, raise otherwise
                 self.imapserver.releaseconnection(imapobj, True)
                 if e.severity == OfflineImapError.ERROR.FOLDER_RETRY:
                     retry = True
                 else: raise
-            finally:
-                self.imapserver.releaseconnection(imapobj)
+            except:
+                # cleanup and raise on all other errors
+                self.imapserver.releaseconnection(imapobj, True)
+                raise
         # 1. Some mail servers do not return an EXISTS response
         # if the folder is empty.  2. ZIMBRA servers can return
         # multiple EXISTS replies in the form 500, 1000, 1500,