MDEV-26767 Server crashes when rename table and alter storage engine

Wrong assertion leftover removed. m_sql_cmd can be allocated by any
ALTER subcommand and before allocation it is checked for NULL first.

mysql-test/main/alter_table.result

@@ -3375,3 +3375,8 @@ drop table t1;
 #
 # End of 10.5 tests
 #
+#
+# MDEV-26767 Server crashes when rename table and alter storage engine
+#
+alter table txxx engine=innodb, rename to tyyy;
+ERROR 42S02: Table 'test.txxx' doesn't exist

mysql-test/main/alter_table.test

@@ -2570,3 +2570,9 @@ drop table t1;
 --echo #
 --echo # End of 10.5 tests
 --echo #
+
+--echo #
+--echo # MDEV-26767 Server crashes when rename table and alter storage engine
+--echo #
+--error ER_NO_SUCH_TABLE
+alter table txxx engine=innodb, rename to tyyy;

sql/sql_lex.cc

@@ -11260,7 +11260,6 @@ bool LEX::stmt_alter_table_exchange_partition(Table_ident *table)
 bool LEX::stmt_alter_table(Table_ident *table)
 {
   DBUG_ASSERT(sql_command == SQLCOM_ALTER_TABLE);
-  DBUG_ASSERT(!m_sql_cmd);
   first_select_lex()->db= table->db;
   if (first_select_lex()->db.str == NULL &&
       copy_db_to(&first_select_lex()->db))