LP:811438 - pbxt crash in start_consistent_snapshot w/o pbxt database

PBXT crashes on start transaction with consistent snapshot if there's no pbxt database.

To repeat (in MariaDB 5.3):
modify suite/pbxt/t/consistent_snapshot.test
as in the patch below, and run it.

=== modified file 'mysql-test/suite/pbxt/t/consistent_snapshot.test'
— mysql-test/suite/pbxt/t/consistent_snapshot.test 2009-04-02 10:03:14 +000
+++ mysql-test/suite/pbxt/t/consistent_snapshot.test 2011-07-16 09:33:18 +000
@@ -4,6 +4,11 @@
drop table if exists t1;
--enable_warnings

+create table t1 (a int);
+insert t1 values (1);
+drop table t1;
+drop database pbxt;
+
connect (con1,localhost,root,,);
connect (con2,localhost,root,,);