Don't assign NULL to boost::shared_ptr

This is already done by the constructor.

Fixes the libreoffic-4-3 build on Ubuntu 12.04.

Change-Id: I7bbec905c0d1efb5670c61c975f36f2e37a74cac
Reviewed-on: https://gerrit.libreoffice.org/11778
Reviewed-by: David Tardon <dtardon@redhat.com>
Tested-by: David Tardon <dtardon@redhat.com>