core/swap: refuse to run swapoff without a device

Issue: https://github.com/systemd/systemd/issues/43442

src/core/swap.c

@@ -873,6 +873,17 @@ static void swap_enter_deactivating(Swap *s) {
 
         assert(s);
 
+        /* A swap unit can be active without ever having been loaded, and hence without a device: an
+         * orphaned unit that still has a swapon running is adopted by swap_coldplug() on purpose, and
+         * swap_sigchld_event() moves it to SWAP_ACTIVE once that process exits successfully. There is
+         * nothing to deactivate for such a unit, and calling swapoff without an argument certainly isn't
+         * it — go to dead, like swap_process_proc_swaps() does when a unit disappears from /proc/swaps. */
+        if (isempty(s->what)) {
+                log_unit_warning(UNIT(s), "Swap unit has no device, assuming it is already deactivated.");
+                swap_enter_dead(s, SWAP_SUCCESS);
+                return;
+        }
+
         s->control_command_id = SWAP_EXEC_DEACTIVATE;
         s->control_command = s->exec_command + SWAP_EXEC_DEACTIVATE;