If a query's end time is before before its start time, the system clock has been turn back
(daylight savings time etc.). When the system clock is changed, we can't tell for certain a
given query was actually slow. We did not protect against logging such a query with a bogus
execution time (resulting from end_time - start_time being negative), and possibly logging it
even though it did not really take long to run.

We now have a sanity check in place.

sql/sql_parse.cc:
  Make sure end time is not before start time - otherwise, we can be SURE the system clock
  was changed in between, but not by how much. In other words, when the clock is changed,
  we don't know how long a query ran, and whether it was slow.