Fix DBCP-383
Avoid NullPointerException and throw an XAException if an attempt is made to commit the current transaction for a connection when no transaction has been started.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/dbcp/branches/DBCP_1_5_x_BRANCH@1564086 13f79535-47bb-0310-9956-ffa450edef68