torture-winbind: Assert that the list of trusted domains is not NULL

torture-winbind: Assert that the list of trusted domains is not NULL

By doing this, we avoid the test being dependent on if the lsa trusted domains tests have run.

Otherwise, we may have a non-null extra_data against the internal winbind, but
only 1 trusted domain (torturedom), but not the local domains that were expected

Signed-off-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Stefan Metzmacher <metze@samba.org>