MDEV-31861 Empty INSERT crashes with innodb_force_recovery=6 or innodb_read_only=ON

ha_innobase::extra(): Do not invoke log_buffer_flush_to_disk()
if high_level_read_only holds.

log_buffer_flush_to_disk(): Remove an assertion that duplicates one
at the start of log_write_up_to().

mysql-test/suite/innodb/r/read_only_recovery.result

@@ -37,6 +37,8 @@ SELECT * FROM t;
 a
 3
 SET GLOBAL innodb_max_purge_lag_wait=0;
+INSERT INTO mysql.innodb_index_stats
+SELECT * FROM mysql.innodb_index_stats LIMIT 0;
 # restart
 SELECT * FROM t;
 a

mysql-test/suite/innodb/t/read_only_recovery.test

@@ -39,6 +39,8 @@ SELECT * FROM t;
 SET TRANSACTION ISOLATION LEVEL READ UNCOMMITTED;
 SELECT * FROM t;
 SET GLOBAL innodb_max_purge_lag_wait=0;
+INSERT INTO mysql.innodb_index_stats
+SELECT * FROM mysql.innodb_index_stats LIMIT 0;
 --let $restart_parameters=
 --source include/restart_mysqld.inc
 SELECT * FROM t;

storage/innobase/handler/ha_innodb.cc

@@ -15797,7 +15797,8 @@ ha_innobase::extra(
 		break;
 	case HA_EXTRA_END_ALTER_COPY:
 		m_prebuilt->table->skip_alter_undo = 0;
-		if (!m_prebuilt->table->is_temporary()) {
+		if (!m_prebuilt->table->is_temporary()
+		    && !high_level_read_only) {
 			log_buffer_flush_to_disk();
 		}
 		break;

storage/innobase/log/log0log.cc

@@ -871,7 +871,6 @@ void log_write_up_to(lsn_t lsn, bool flush_to_disk, bool rotate_key,
 @param sync  whether to wait for a durable write to complete */
 void log_buffer_flush_to_disk(bool sync)
 {
-  ut_ad(!srv_read_only_mode);
   log_write_up_to(log_sys.get_lsn(std::memory_order_acquire), sync);
 }