merge SVN commit 1102400 by winterz:

fix a crash when deleting a folder in the following test case:

1. Create a subfolder x2 of x1.
2. Copy all mails form x1 to x2.
3. Sync and while syncing delete folder x1

kolab/issue3902

svn path=/tags/kdepim/enterprise35.0.20100312.1102367/kdepim/; revision=1102405