Before invoking fil_space_t::acquire(), check space->is_stopping()
to ensure that the tablespace actually is accessible. This fixes
a regression introduced by MDEV-15983.

fil_space_next(): Remove some duplicated code for prev_space==NULL,
and check is_stopping() also on the first tablespace.

i_s_tablespaces_encryption_fill_table(),
i_s_tablespaces_scrubbing_fill_table(): Check is_stopping().