If the system time is adjusted back during a query execution
(resulting in the end time being earlier than the start time)
the code that prints to the slow query log gets confused and 
prints unsigned negative numbers.
            
Fixed by not logging the statements that would have negative
execution time due to time shifts.
No test case since this would involve changing the system time.

sql/sql_parse.cc:
  Bug #35396: don't log queries with negative execution times 
  (due to shifts in system time).