Only call spa_man_trim_taskq_destroy() when needed

Long zloop runs were occasionally hitting the
"ASSERT(spa->spa_man_trim_taskq != NULL)" in spa_man_trim_taskq_destroy().
It's not clear to me how this was happening because the only place
"spa->spa_man_trim_taskq" is cleared is in spa_man_trim_taskq_destroy()
itself which is only called from the (single) spa_async_thread() or
from spa_unload().

To that end, this commit adds a non-NULL check in spa_async_thread()
analagous to the tests which were added when support for stopping/starting
the auto trim taskqs were added to spa_async_thread().

NOTE: Yes, that means I consider this to be a band-aid.

module/zfs/spa.c

@@ -7260,7 +7260,8 @@ spa_async_thread(void *arg)
 	 */
 	if (tasks & SPA_ASYNC_MAN_TRIM_TASKQ_DESTROY) {
 		mutex_enter(&spa->spa_man_trim_lock);
-		spa_man_trim_taskq_destroy(spa);
+		if (spa->spa_man_trim_taskq != NULL)
+			spa_man_trim_taskq_destroy(spa);
 		mutex_exit(&spa->spa_man_trim_lock);
 	}
 	if (tasks & SPA_ASYNC_AUTO_TRIM_TASKQ_CREATE) {