resolve: do not trigger assertion on exit

resolve: do not trigger assertion on exit

By making assert_return() critical, we observe the following:
---
 Program received signal SIGABRT, Aborted.
 0x00007f01320b0884 in __pthread_kill_implementation () from /lib64/libc.so.6
 (gdb) bt
 #0  0x00007f01320b0884 in __pthread_kill_implementation ()
    from /lib64/libc.so.6
 #1  0x00007f013205fafe in raise () from /lib64/libc.so.6
 #2  0x00007f013204887f in abort () from /lib64/libc.so.6
 #3  0x00007f01338d02d6 in log_assert_failed (
     text=0x7f01340009e0 "e->state != SD_EVENT_FINISHED",
     file=0x7f0133fff403 "src/libsystemd/sd-event/sd-event.c", line=1399,
     func=0x7f01340045a0 <__func__.148> "sd_event_add_time")
     at ../src/basic/log.c:948
 #4  0x00007f01338d0457 in log_assert_failed_return (
     text=0x7f01340009e0 "e->state != SD_EVENT_FINISHED",
     file=0x7f0133fff403 "src/libsystemd/sd-event/sd-event.c", line=1399,
     func=0x7f01340045a0 <__func__.148> "sd_event_add_time")
     at ../src/basic/log.c:967
 #5  0x00007f0133c7ed83 in sd_event_add_time (e=0x617000022280,
     ret=0x610000007e98, clock=7, usec=24054941030, accuracy=0,
     callback=0x4625b4 <on_announcement_timeout>, userdata=0x610000007e40)
     at ../src/libsystemd/sd-event/sd-event.c:1399
 #6  0x00007f0133c7f725 in sd_event_add_time_relative (e=0x617000022280,
     ret=0x610000007e98, clock=7, usec=1000000, accuracy=0,
     callback=0x4625b4 <on_announcement_timeout>, userdata=0x610000007e40)
     at ../src/libsystemd/sd-event/sd-event.c:1462
 #7  0x0000000000464cac in dns_scope_announce (scope=0x610000007e40, goodbye=true) at ../src/resolve/resolved-dns-scope.c:1530
 #8  0x0000000000504d08 in link_free (l=0x612000023d40) at ../src/resolve/resolved-link.c:83
 #9  0x000000000052dbbd in manager_free (m=0x619000000a80) at ../src/resolve/resolved-manager.c:697
 #10 0x0000000000562328 in manager_freep (p=0x7f012f800040) at ../src/resolve/resolved-manager.h:198
 #11 0x000000000056315a in run (argc=1, argv=0x7fff22b06468) at ../src/resolve/resolved.c:25
 #12 0x0000000000563284 in main (argc=1, argv=0x7fff22b06468) at ../src/resolve/resolved.c:99
---
Prompted by https://github.com/systemd/systemd/pull/30049#issuecomment-1844087965.