Fix a crash in the monitor.

This patch set fixes a crash in the monitor loop in case of receiving an STH
with just an increased timestamp (i.e. tree size does not change).

It also logs non-critical network problems while fetching STHs.

This is a patch I pulled from here: https://codereview.appspot.com/72580043/
Consistency:

const CheckResult sanity(CheckSTHSanity(old_sth, new_sth));

Though I prefer = to () myself.