valgrind: check return of convertDateTime before using fDate

lots of spew in VALGRIND=memcheck make CppunitTest_writerperfect_calc

Change-Id: I8b95e95218beefe19e881c9d055323d0abec49c7
(cherry picked from commit 3150ae30415c1253258829dd3d39c015e75c6171)
Reviewed-on: https://gerrit.libreoffice.org/12403
Reviewed-by: David Tardon <dtardon@redhat.com>
Tested-by: David Tardon <dtardon@redhat.com>