v.20.11Bug Fix

Fix SIGSEGV with merge_tree_min_rows_for_concurrent_read/merge_tree_min_bytes_for_concurrent_read configuration settings

Fix SIGSEGV with merge_tree_min_rows_for_concurrent_read/merge_tree_min_bytes_for_concurrent_read=0/UINT64_MAX. #19528 (Azat Khuzhin).