Fix the crashes when viewing messages.

The fix in kmacctimap.cpp is to prevent double incrementing which moves us past the end of the list, and then we dereference it, then crashing.

The fix in kmmainwidget.cpp is to make sure that FolderJobs (and ergo, ImapJobs) always get an account to remove themselves from when it's time for them to die. (The account is referenced either through the destination folder or the source folder).