Fix TROVE-2018-004: bad consensus can trigger null pointer crash.

When checking their own versions against the subprotocol versions listed in a consensus document, Tor instances could be made to crash if the consensus was incorrectly formatted.

This is a low-severity bug, since it can only be exploited by corrupting a majority of directory authorities. (And any attacker who can do that, can do far worse.)

We're tracking this one as TROVE-2018-004. It was present in 0.2.9.4-alpha and later. It is fixed in 0.2.9.15, 0.3.1.10, 0.3.2.10, and 0.3.3.3-alpha.