xrootd: Upgrade to xrootd4j v1.2.4

Contains the following changes over v1.2.3:

42101c6 xrootd4j: Make errors null terminated
5ae3a44 Fix read beyond end of file

These changes improve compatibility with the "new xrootd client".

Target: trunk
Request: 2.8
Request: 2.7
Request: 2.6
Ticket: http://rt.dcache.org/Ticket/Display.html?id=8156
Acked-by: Tigran Mkrtchyan <tigran.mkrtchyan@desy.de>
Patch: http://rb.dcache.org/r/6557/