Fixed a NullPointerException that is causing the following error message:
(ERROR) [03/Dec/2013:12:10:55 +0100] category=CORE severity=FATAL_ERROR msgID=327820 msg=An uncaught exception during processing for thread Change number indexer has caused it to terminate abnormally. The stack trace for that exception is: NullPointerException (ChangeNumberIndexer.java:337 ChangeNumberIndexer.java:324 ChangeNumberIndexer.java:266 ChangeNumberIndexer.java:355)

ChangeNumberIndexer.java:
In getPrecedingCSN(), handled the null case.