Make sure _shutdown is always set

Very often event logs don't contain a CapDelete or Shutdown event (even
when you exit the program properly). When this is the case
ghc-events-analyze crashes with the message "_shutdown not set". This is
annoying since the whole event log could in principle be visualized.

This patch just marks the last event time as the shutdown time.
Merge pull request #1 from basvandijk/fix-shutdown-not-set-error

Make sure _shutdown is always set