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/
Good call.

The () vs = thing is that I like to make sure there's no temporary created and *then* copied, even though I'm being a bit silly, since in reality using = gets optimised to the same, normally. Just a quirk of mine...