gnutls_pkcs11_obj_list_import_url4: always return an initialized pointer

When returning success, but no elements, gnutls_pkcs11_obj_list_import_url4, could have returned zero number of elements with a pointer that was uninitialized. Ensure that an initialized (i.e., null in that case), pointer is always returned. Reported by Jeremy Harris.

Signed-off-by: Nikos Mavrogiannopoulos nmav@redhat.com