Don't skip row_end if it wasn't set explicitly.

Also another segfault was caused by accessing rpl_write_set on slave during
the row update/delete.
The reason was a default_column_bitmaps() call, which also sets
rpl_write_set to NULL.
Previously, the related behavior was changed in commit afd3ee97ad, where
one such call was removed from Update_rows_log_event::do_exec_row, but the
same one was mistakenly left in Delete_rows_log_event. Now it's also
removed.