Fix SIGSEGV for External GROUP BY with Overflow Row in ClickHouse

Fix SIGSEGV for external GROUP BY and overflow row (i.e. queries like SELECT FROM GROUP BY WITH TOTALS SETTINGS max_bytes_before_external_group_by>0, max_rows_to_group_by>0, group_by_overflow_mode='any', totals_mode='before_having'). #23962 (Azat Khuzhin).