`log_prefix_` maybe getting weird output from strftime

legacy/trac#19408 (moved) purports to show tor log lines. The date format in them is really strange:

6/13/2016 23:53:24 PM.700
6/13/2016 23:53:26 PM.200
6/14/2016 0:07:45 AM.200
6/14/2016 0:07:45 AM.200

There's a date string followed by a dot and three digits' worth of fractional seconds. The format doesn't match what's specified in log_prefix_, which is "%b %d %H:%M:%S". https://gitweb.torproject.org/tor.git/tree/src/common/log.c?id=tor-0.2.7.6#n221

I don't see how this can happen, but I thought some one in core tor might want to take a look. Maybe the user has some kind of smart copy-paste that's mangling dates.