fix the method name

it's always assert_<statement>,
not <do something>_if_<condition>

sql/sql_class.h

@@ -4084,12 +4084,12 @@ inline void add_to_active_threads(THD *thd)
 /*
   This should be called when you want to delete a thd that was not
   running any queries.
-  This function will assert if the THD was not linked.
+  This function will assert that the THD is linked.
 */
 
 inline void unlink_not_visible_thd(THD *thd)
 {
-  thd->assert_if_linked();
+  thd->assert_linked();
   mysql_mutex_lock(&LOCK_thread_count);
   thd->unlink();
   mysql_mutex_unlock(&LOCK_thread_count);

sql/sql_list.h

@@ -660,7 +660,7 @@ struct ilink
     if (next) next->prev=prev;
     prev=0 ; next=0;
   }
-  inline void assert_if_linked()
+  inline void assert_linked()
   {
     DBUG_ASSERT(prev != 0 && next != 0);
   }