Ported commit 1f0a8a384790513c6686e96189ba243b18b675db from castor/master

CASTOR-5419: Ceph aborts when writing a 0-sized buffer

Prevented writes of zero size in DiskWriteTask. They happen
when we read from tape a file aligned with the memory block size.

tapeserver/castor/tape/tapeserver/daemon/DiskWriteTask.cpp

@@ -97,7 +97,8 @@ bool DiskWriteTask::execute(RecallReportPacker& reporter,cta::log::LogContext&
         // Write the data.
         currentErrorToCount = "Error_diskWrite";
         m_stats.dataVolume+=mb->m_payload.size();
-        mb->m_payload.write(*writeFile);
+        if (mb->m_payload.size())
+          mb->m_payload.write(*writeFile);
         m_stats.readWriteTime+=localTime.secs(cta::utils::Timer::resetCounter);
         
         checksum = mb->m_payload.adler32(checksum);