two crashes in the TC_LOG_MMAP:

1. don't forget to initialize page->ptr
2. don't signal active->cond, if active is NULL

mysql-test/suite/pbxt/r/pbxt_xa.result

+drop table if exists t1, t2;
+CREATE TABLE t1 (a INT PRIMARY KEY) ENGINE=innodb;
+CREATE TABLE t2 (b INT PRIMARY KEY) ENGINE=pbxt;
+BEGIN;
+SELECT @@log_bin;
+@@log_bin
+0
+INSERT INTO t1 VALUES (1);
+INSERT INTO t2 VALUES (2);
+COMMIT;
+select * from t1;
+a
+1
+select * from t2;
+b
+2
+drop table t1, t2;
+drop database pbxt;

mysql-test/suite/pbxt/t/pbxt_xa.test

+-- source include/have_innodb.inc
+
+--disable_warnings
+drop table if exists t1, t2;
+--enable_warnings
+
+#
+# bug lp:544173, xa crash with two 2pc-capable storage engines without binlog
+#
+
+CREATE TABLE t1 (a INT PRIMARY KEY) ENGINE=innodb;
+CREATE TABLE t2 (b INT PRIMARY KEY) ENGINE=pbxt;
+BEGIN;
+# verify that binlog is off
+SELECT @@log_bin;
+INSERT INTO t1 VALUES (1);
+INSERT INTO t2 VALUES (2);
+COMMIT;
+select * from t1;
+select * from t2;
+drop table t1, t2;
+drop database pbxt;
+

sql/log.cc

@@ -5387,7 +5387,7 @@ int TC_LOG_MMAP::open(const char *opt_name)
     pg->state=POOL;
     pthread_mutex_init(&pg->lock, MY_MUTEX_INIT_FAST);
     pthread_cond_init (&pg->cond, 0);
-    pg->start=(my_xid *)(data + i*tc_log_page_size);
+    pg->ptr= pg->start=(my_xid *)(data + i*tc_log_page_size);
     pg->size=pg->free=tc_log_page_size/sizeof(my_xid);
     pg->end=pg->start + pg->size;
   }
@@ -5622,6 +5622,14 @@ int TC_LOG_MMAP::sync()
   /* marking 'syncing' slot free */
   pthread_mutex_lock(&LOCK_sync);
   syncing=0;
+  /*
+    we check the "active" pointer without LOCK_active. Still, it's safe -
+    "active" can change from NULL to not NULL any time, but it
+    will take LOCK_sync before waiting on active->cond. That is, it can never
+    miss a signal.
+    And "active" can change to NULL only after LOCK_sync, so this is safe too.
+  */
+  if (active)
     pthread_cond_signal(&active->cond);      // wake up a new syncer
   pthread_mutex_unlock(&LOCK_sync);
   return err;