Fixed some compilation warnings/errors

sql/item_sum.cc

@@ -2046,7 +2046,7 @@ double Item_sum_std::val_real()
 {
   DBUG_ASSERT(fixed == 1);
   double nr= Item_sum_variance::val_real();
-  if (isnan(nr))
+  if (std::isnan(nr))
   {
     /*
       variance_fp_recurrence_next() can overflow in some cases and return "nan":

sql/mysqld.cc

@@ -3043,9 +3043,8 @@ static bool cache_thread(THD *thd)
         Create new instrumentation for the new THD job,
         and attach it to this running pthread.
       */
-      PSI_thread *psi= PSI_CALL_new_thread(key_thread_one_connection,
-                                                   thd, thd->thread_id);
-      PSI_CALL_set_thread(psi);
+      PSI_CALL_set_thread(PSI_CALL_new_thread(key_thread_one_connection,
+                                              thd, thd->thread_id));
 
       /* reset abort flag for the thread */
       thd->mysys_var->abort= 0;