... to take into account possible differences of results of dynamic_cast
vs static_cast; the change casts dynamically, as before the commit, and
only adds asserts on the result of the cast.

Thanks to sberg for pointing my mistake out!

Change-Id: Ib77d443e5a858e744f369f58542de603f948fd1c
Reviewed-on: https://gerrit.libreoffice.org/70274
Tested-by: Jenkins
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>