purging apt-xapian-index fails because it tries to delete non-existing directory

[Sebastian Keller]

When purging the package I got an error-message when deleting /var/lib/apt-xapian-index. This directory did not exist on my system (possibly because I only had this package installed for a short time, as it was pulled in as a suggestion of synaptic).
I was able to complete purging by creating the according directory before this operation. So I think the postrm script should check whether this directory exists before it tries to remove it.