prepared statement execute


mysql-test/r/sp.result:
  Test results fixed: a fix for Bug#15392
mysql-test/t/sp.test:
  A test case for Bug#15392 "Server crashes during prepared
  statement execute". No test case for error in
  Item_func_set_user_var::update as the only possible one is OOM.
sql/sp_head.cc:
  A fix for Bug#15392 "Server crashes during prepared statement
  execute": the bug was caused by mysql_change_db() call
  which was overwriting the error state of 'ret'.
  Later in the code, suv->fix_fields() would discover
  thd->net.report_error and return it without completing
  its work. As the return value of fix_fields() was ignored,
  the server would afterwards crash in suv->update().
  The fix makes sure that a possible internal error
  is raised in reset_lex_and_exec_core and then is
  handled in sp_head::execute_procedure.