Panic if all TAL files are deleted after running routinator init

[ximon18]

With Routinator 0.5.1 (Docker) after init -f --accept-arin-rpa and docker run -d:

> docker exec -it routinator /bin/sh
/ $ rm /home/routinator/.rpki-cache/tals/*
/ $ exit

> docker restart routinator

> docker logs routinator
...
routinator    | Starting RTR listener.
routinator    | thread 'main' panicked at 'capacity must be non-zero', /root/.cargo/registry/src/github.com-1ecc6299db9ec823/crossbeam-queue-0.1.2/src/array_queue.rs:103:9
routinator    | note: Run with `RUST_BACKTRACE=1` environment variable to display a backtrace.

My intent was to have just a single TAL file for testing Krill, but I forgot to put the Krill TAL file in the tals/ directory before restarting Routinator and thus had zero TAL files present.