Fix for MDEV-15812 Assert in SEQUENCE when forcing STATEMEMT format

The bug was the we copied the lock type to the underlying engine even when
external_lock failed.

mysql-test/suite/sql_sequence/replication_drop.result

+CREATE SEQUENCE seq ENGINE=InnoDB;
+SET SESSION TRANSACTION ISOLATION LEVEL READ COMMITTED;
+INSERT INTO seq VALUES (1,1,100,1,1,1,1,1);
+ERROR HY000: Cannot execute statement: impossible to write to binary log since BINLOG_FORMAT = STATEMENT and at least one table uses a storage engine limited to row-based logging. InnoDB is limited to row-logging when transaction isolation level is READ COMMITTED or READ UNCOMMITTED.
+DROP SEQUENCE seq;

mysql-test/suite/sql_sequence/replication_drop.test

+#
+# Test for MDEV-15812
+# Assertion `m_lock_type == 2' failed in
+# handler::~handler on dropping a sequence after
+# ER_BINLOG_STMT_MODE_AND_ROW_ENGINE
+#
+
+--source include/have_innodb.inc
+--source include/have_binlog_format_statement.inc
+
+CREATE SEQUENCE seq ENGINE=InnoDB;
+SET SESSION TRANSACTION ISOLATION LEVEL READ COMMITTED;
+--error ER_BINLOG_STMT_MODE_AND_ROW_ENGINE
+INSERT INTO seq VALUES (1,1,100,1,1,1,1,1);
+
+# Cleanup
+DROP SEQUENCE seq;

sql/ha_sequence.cc

@@ -322,7 +322,8 @@ int ha_sequence::external_lock(THD *thd, int lock_type)
     Copy lock flag to satisfy DBUG_ASSERT checks in ha_* functions in
     handler.cc when we later call it with file->ha_..()
   */
-  file->m_lock_type= lock_type;
+  if (!error)
+    file->m_lock_type= lock_type;
   return error;
 }