fix: nil: Stop the timer when canceling the last fetch

When canceling the last fetch, we also need to stop the fctx_expired
timer from possibly firing between the fctx_shutdown() call and the
fetch being actually destroyed along with the timer.

Closes #5136

Merge branch '5136-stop-timer-when-canceling-last-fetch-9.18' into 'bind-9.18'

See merge request isc-projects/bind9!9988