MDEV-5374 main.func_time fails with valgrind warning

"Conditional jump or move depends on uninitialised"
in Item_time_typecast::get_date.

Adding "null_value" into the debug assert condition.

sql/item_func.h

@@ -159,7 +159,8 @@ class Item_func :public Item_result_field
   inline bool get_arg0_time(MYSQL_TIME *ltime)
   {
     null_value= args[0]->get_time(ltime);
-    DBUG_ASSERT(ltime->time_type != MYSQL_TIMESTAMP_TIME || ltime->day == 0);
+    DBUG_ASSERT(null_value ||
+                ltime->time_type != MYSQL_TIMESTAMP_TIME || ltime->day == 0);
     return null_value;
   }
   bool is_null() {