Initialize and test properly when cleaning up, to avoid crash in some error cases.


mysql-test/r/sp-error.result:
  Test case for BUG#2259
mysql-test/t/sp-error.test:
  Test case for BUG#2259
sql/sp_rcontext.cc:
  Cleanup in a more controlled way to avoid crashes in some error cases.
  Remove unused variable.
sql/sp_rcontext.h:
  Initialize member var, for cleanup tests.