@@optimizer_switch switch from no_xxx to xxx={on|off} syntax:

- Fix valgrind warning on attempt to run a "SET optimizer_switch=number" statement.
  Need to call c_ptr_safe() as strings returned by non-string items are not 
  necessarily null-terminated.

sql/set_var.cc

@@ -3990,7 +3990,7 @@ bool sys_var_thd_optimizer_switch::check(THD *thd, set_var *var)
                                optimizer_switch_typelib.count, 
                                thd->variables.optimizer_switch,
                                global_system_variables.optimizer_switch,
-                               res->c_ptr(), res->length(), NULL,
+                               res->c_ptr_safe(), res->length(), NULL,
                                &error, &error_len, &not_used);
   if (error_len)
   {