DB-762 create tokudb trx when needed in tokudb::start_stmt

mysql-test/suite/tokudb.bugs/r/db762.result

+drop table if exists t1,t2;
+create table t1 (x int) engine=innodb;
+lock table t1 read;
+create temporary table t2 (x int) engine=tokudb;
+insert into t2 values (1);
+unlock tables;
+drop table t1, t2;

mysql-test/suite/tokudb.bugs/t/db762.test

+# test for DB-762 and DB-767
+source include/have_tokudb.inc;
+source include/have_innodb.inc;
+disable_warnings;
+drop table if exists t1,t2;
+enable_warnings;
+create table t1 (x int) engine=innodb;
+lock table t1 read;
+create temporary table t2 (x int) engine=tokudb;
+insert into t2 values (1);
+unlock tables;
+drop table t1, t2;
+

storage/tokudb/ha_tokudb.cc

@@ -6227,7 +6227,11 @@ int ha_tokudb::start_stmt(THD * thd, thr_lock_type lock_type) {
 
     int error = 0;
     tokudb_trx_data *trx = (tokudb_trx_data *) thd_get_ha_data(thd, tokudb_hton);
-    DBUG_ASSERT(trx);
+    if (!trx) {
+        error = create_tokudb_trx_data_instance(&trx);
+        if (error) { goto cleanup; }
+        thd_set_ha_data(thd, tokudb_hton, trx);
+    }
 
     /*
        note that trx->stmt may have been already initialized as start_stmt()