MDEV-33245 SIGSEGV in wsrep_check_sequence

The segfault in wsrep_check_sequence is due to a
null pointer deference on:
  db_type= thd->lex->create_info.db_type->db_type;

Where create_info.db_type is null. This occured under
a used_engine==true condition which is set in the calling
function based on create_info.used_fields==HA_CREATE_USED_ENGINE.

However the create_info.used_fields was a left over
from the parsing of the previous failed CREATE TABLE where
because of its failure, db_type wasn't populated.

This is corrected by cleaning the create_info when we start
to parse ALTER SEQUENCE statements.

Other paths to wsrep_check_sequence is via CREATE SEQUENCE
and CREATE TABLE LIKE which both initialize the create_info
correctly.

mysql-test/suite/galera/r/galera_sequences.result

@@ -314,3 +314,14 @@ NEXTVAL(t)
 connection node_1;
 DROP TABLE t1;
 DROP SEQUENCE t;
+
+MDEV-33245 SIGSEGV in wsrep_check_sequence | Sql_cmd_alter_sequence::execute
+
+CREATE TABLE t (a INT) ENGINE=InnoDB;
+INSERT INTO t VALUES (0);
+CREATE TABLE t1 (c VARCHAR) ENGINE=InnoDB;
+ERROR 42000: You have an error in your SQL syntax; check the manual that corresponds to your MariaDB server version for the right syntax to use near ') ENGINE=InnoDB' at line 1
+ALTER SEQUENCE IF EXISTS t MINVALUE=1;
+ERROR 42000: This version of MariaDB doesn't yet support 'CACHE without INCREMENT BY 0 in Galera cluster'
+DROP TABLE t;
+End of 10.5 tests

mysql-test/suite/galera/t/galera_sequences.test

@@ -341,3 +341,18 @@ SELECT NEXTVAL(t);
 --connection node_1
 DROP TABLE t1;
 DROP SEQUENCE t;
+
+--echo
+--echo MDEV-33245 SIGSEGV in wsrep_check_sequence | Sql_cmd_alter_sequence::execute
+--echo
+
+CREATE TABLE t (a INT) ENGINE=InnoDB;
+INSERT INTO t VALUES (0);
+
+--error ER_PARSE_ERROR
+CREATE TABLE t1 (c VARCHAR) ENGINE=InnoDB;
+--error ER_NOT_SUPPORTED_YET
+ALTER SEQUENCE IF EXISTS t MINVALUE=1;
+
+DROP TABLE t;
+--echo End of 10.5 tests

sql/sql_yacc.yy

@@ -7472,6 +7472,7 @@ alter:
           {
             LEX *lex= Lex;
             lex->sql_command= SQLCOM_ALTER_SEQUENCE;
+            lex->create_info.init();
             DBUG_ASSERT(!lex->m_sql_cmd);
             if (Lex->main_select_push())
               MYSQL_YYABORT;