implement some crash guards to in formatICalInviation.
implement some crash guards to in formatICalInviation.

for kolab/issue3694

svn path=/branches/kdepim/enterprise/kdepim/; revision=979860

lefou/kdepim-noakonadi,lefou/kdepim-noakonadi,lefou/kdepim-noakonadi,lefou/kdepim-noakonadi,lefou/kdepim-noakonadi,lefou/kdepim-noakonadi