prime number test example.


mysql-test/r/sp.result:
  New prime number example. (Good for future benchmarkings too.)
mysql-test/t/sp.test:
  New prime number example. (Good for future benchmarkings too.)
sql/sql_class.cc:
  Reset row_count in select_dumpvar when preparing, to make it reentrant as a lex member.
sql/sql_select.cc:
  Don't delete result if it's a lex member (since lex needs to be reentrant).