[ci skip] Fix transactions for the ibase driver

Reported via the forums: http://forum.codeigniter.com/thread-64559.html
diff --git a/system/database/drivers/ibase/ibase_driver.php b/system/database/drivers/ibase/ibase_driver.php
index cbc1022..c1055c1 100644
--- a/system/database/drivers/ibase/ibase_driver.php
+++ b/system/database/drivers/ibase/ibase_driver.php
@@ -126,7 +126,7 @@
 	 */
 	protected function _execute($sql)
 	{
-		return ibase_query($this->conn_id, $sql);
+		return ibase_query(isset($this->_ibase_trans) ? $this->_ibase_trans : $this->conn_id, $sql);
 	}
 
 	// --------------------------------------------------------------------