TROVE-2018-001: null-pointer crash in directory authority protocol list code

The subprotocol implementation in 0.2.9.4-alpha had a bug where an incorrectly formatted relay descriptor could cause directory servers to crash when they tried to vote about it. This does not affect relays or clients, since they do not try to vote.

Tracked as TROVE-2018-001 and CVE-2018-0490.