In compaction_manager::really_do_stop we check whether _tasks list
is empty after the compactions are stopped. However, a new task may
still sneak in, causing the assertion failure. Such a task won't
be there for long - module::make_task will fail as the module is
already stopped.
Move the assertion, that checks if _tasks is empty, after the
compaction_states' gates are closed.
Fixes: #25806.
(cherry picked from commit 97c77d7cd5)
7c847d76f6