BUG#6642: Stored procedure crash if expression with set function
  BUG#7013: Stored procedure crash if group by ... with rollup
  BUG#7743: 'Lost connection to MySQL server during query' on Stored Procedure
  BUG#7992: SELECT .. INTO variable .. within Stored Procedure crashes
            the server
  BUG#8116: calling simple stored procedure twice in a row results
            in server crash

Rolling back the item change list after each substatement in a procedure
fixed the failing assert().


mysql-test/r/sp.result:
  New test cases for BUG#6642, 7013, 7743, 7992 and 8116.
mysql-test/t/sp.test:
  New test cases for BUG#6642, 7013, 7743, 7992 and 8116.
sql/sp_head.cc:
  Roll back the item change list after each sub statement.