Revert "loop: remove destroy list"

This reverts commit c474846c42967c44db069a23b76a29da6f496f33.
In addition, `s->loop` is also checked before dispatching a source.

The destroy list is needed in the presence of threads. The
issue is that a source may be destroyed between `epoll_wait()`
returning and thread loop lock being acquired. If this
source is active, then a use-after-free will be triggered
when the thread loop acquires the lock and starts dispatching
the sources.

  thread 1                       thread 2
 ----------                     ----------
                                loop_iterate
                                  spa_loop_control_hook_before
                                    // release lock

 pw_thread_loop_lock

                                  spa_system_pollfd_wait
                                    // assume it returns with source A

 pw_loop_destroy_source(..., A)
  // frees storage of A

 pw_thread_loop_unlock
                                  spa_loop_control_hook_after
                                    // acquire the lock

                                  for (...) {
                                    struct spa_source *s = ep[i].data;
                                    s->rmask = ep[i].events;
                                      // use-after-free if `s` refers to
                                      // the previously freed `A`