NullPointerException in XAPool.containsXAResourceHolderMatchingGtrid()

While testing BTM on a somewhat slow server, we noticed a NullPointerException in XAPool.containsXAResourceHolderMatchingGtrid() that was breaking stuff.
It seems like nobody was checking if bitronixXid was null before calling a method on it.

I've included a patch which I believe fixes this problem but I'm not completely sure if it is the correct action to take because I don't know how BTM works. Essentially, if bitronixXid is null, it just skips that resource instead of causing a NullPointerException.

Here's the relevant stack trace:

Caused by: java.sql.SQLException: unable to get a connection from pool of a PoolingDataSource containing an XAPool of resource jdbc/DS with 5 connection(s) (3 still available)
at bitronix.tm.resource.jdbc.PoolingDataSource.getConnection(PoolingDataSource.java:109)
at org.hibernate.connection.DatasourceConnectionProvider.getConnection(DatasourceConnectionProvider.java:92)
at org.hibernate.jdbc.ConnectionManager.openConnection(ConnectionManager.java:446)
... 144 more
Caused by: java.lang.NullPointerException
at bitronix.tm.resource.common.XAPool.containsXAResourceHolderMatchingGtrid(XAPool.java:305)
at bitronix.tm.resource.common.XAPool.getNotAccessible(XAPool.java:286)
at bitronix.tm.resource.common.XAPool.getConnectionHandle(XAPool.java:96)
at bitronix.tm.resource.common.XAPool.getConnectionHandle(XAPool.java:71)
at bitronix.tm.resource.jdbc.PoolingDataSource.getConnection(PoolingDataSource.java:105)
... 146 more