In BENCHMARK(count, expr), count could overflow/wrap-around.
Patch changes to a sufficiently large data-type. Adds a warning
for negative count values.


mysql-test/r/func_str.result:
  show that a negative 'count' for BENCHMARK(count, expr)
  throws a warning and returns NULL.
mysql-test/t/func_str.test:
  show that a negative 'count' for BENCHMARK(count, expr)
  throws a warning and returns NULL.
sql/item_func.cc:
  use ulonglong rather than ulong in BENCHMARK(count, expr)
  throw warning on negative 'count'; return SQL-NULL.