fix crash on loading fdo70673-1.ods

rStr is NaN

Change-Id: I8206ab322a08d558cf5f05750efd700875b2134c
(cherry picked from commit 12ec403c547cca4d7e5f73ae3ef58ff8faf3721e)
Reviewed-on: https://gerrit.libreoffice.org/10109
Reviewed-by: Eike Rathke <erack@redhat.com>
Tested-by: Eike Rathke <erack@redhat.com>