executing sub-statements (selects) prevents crashes intermittent
crashes.
Also fixed bug in sql_yacc.yy (generated a hpop instruction when
not needed).


mysql-test/r/sp.result:
  Test-case for BUG#822.
mysql-test/t/sp.test:
  Test-case for BUG#822.
sql/sp_head.cc:
  Fixed BUG#822; needed to copy and clear some stuff when calling sub-statements.
sql/sql_yacc.yy:
  Only generate hpop instruction when we actually have handlers.