MDEV-21341: Fix UBSAN failures, part #5

Item_cond inherits from Item_args but doesn't store its arguments
as function arguments, which means it has zero arguments.
Don't call memcpy in this case.

sql/item_func.cc

@@ -109,6 +109,7 @@ Item_args::Item_args(THD *thd, const Item_args *other)
     arg_count= 0;
     return;
   }
+  if (arg_count)
     memcpy(args, other->args, sizeof(Item*) * arg_count);
 }